Internet Small Computer System Interface (iSCSI) Protocol

(Consolidated)

Abstract

This document describes a transport protocol for SCSI that works on
top of TCP. The iSCSI protocol aims to be fully compliant with the
standardized SCSI Architecture Model (SAM-2). RFC 3720 defined the
original iSCSI protocol. RFC 3721 discusses iSCSI naming examples
and discovery techniques. Subsequently, RFC 3980 added an additional
naming format to the iSCSI protocol. RFC 4850 followed up by adding
a new public extension key to iSCSI. RFC 5048 offered a number of
clarifications as well as a few improvements and corrections to the
original iSCSI protocol.

This document obsoletes RFCs 3720, 3980, 4850, and 5048 by
consolidating them into a single document and making additional
updates to the consolidated specification. This document also
updates RFC 3721. The text in this document thus supersedes the text
in all the noted RFCs wherever there is a difference in semantics.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7143.

Copyright Notice

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described in the Simplified BSD License.

1. Introduction

The Small Computer System Interface (SCSI) is a popular family of
protocols for communicating with I/O devices, especially storage
devices. SCSI is a client-server architecture. Clients of a SCSI
interface are called "initiators". Initiators issue SCSI "commands"
to request services from components -- logical units of a server
known as a "target". A "SCSI transport" maps the client-server SCSI
protocol to a specific interconnect. An initiator is one endpoint of
a SCSI transport, and a target is the other endpoint.

The SCSI protocol has been mapped over various transports, including
Parallel SCSI, Intelligent Peripheral Interface (IPI), IEEE 1394
(FireWire), and Fibre Channel. These transports are I/O-specific and
have limited distance capabilities.

The iSCSI protocol defined in this document describes a means of
transporting SCSI packets over TCP/IP, providing for an interoperable
solution that can take advantage of existing Internet infrastructure,
Internet management facilities, and address distance limitations.

2.2. Definitions

Alias: An alias string can also be associated with an iSCSI node.
The alias allows an organization to associate a user-friendly
string with the iSCSI name. However, the alias string is not a
substitute for the iSCSI name.

CID (connection ID): Connections within a session are identified by
a connection ID. It is a unique ID for this connection within the
session for the initiator. It is generated by the initiator and
presented to the target during Login Requests and during logouts
that close connections.

Connection: A connection is a TCP connection. Communication
between the initiator and target occurs over one or more TCP
connections. The TCP connections carry control messages, SCSI
commands, parameters, and data within iSCSI Protocol Data Units
(iSCSI PDUs).

I/O Buffer: An I/O Buffer is a buffer that is used in a SCSI read
or write operation so SCSI data may be sent from or received into
that buffer. For a read or write data transfer to take place for a
task, an I/O Buffer is required on the initiator and at least one
is required on the target.

INCITS: "INCITS" stands for InterNational Committee for Information
Technology Standards. The INCITS has a broad standardization scope
within the field of Information and Communications Technologies
(ICT), encompassing storage, processing, transfer, display,
management, organization, and retrieval of information. INCITS
serves as ANSI's Technical Advisory Group for the ISO/IEC Joint
Technical Committee 1 (JTC 1). See <http://www.incits.org>.

iSCSI Device: An iSCSI device is a SCSI device using an iSCSI
service delivery subsystem. The Service Delivery Subsystem is
defined by [SAM2] as a transport mechanism for SCSI commands and
responses.

iSCSI Initiator Name: The iSCSI Initiator Name specifies the
worldwide unique name of the initiator.

iSCSI Initiator Node: An iSCSI initiator node is the "initiator"
device. The word "initiator" has been appropriately qualified as
either a port or a device in the rest of the document when the
context is ambiguous. All unqualified usages of "initiator" refer
to an initiator port (or device), depending on the context.

iSCSI Layer: This layer builds/receives iSCSI PDUs and
relays/receives them to/from one or more TCP connections that form
an initiator-target "session".

iSCSI Name: This is the name of an iSCSI initiator or iSCSI target.

iSCSI Node: The iSCSI node represents a single iSCSI initiator or
iSCSI target, or a single instance of each. There are one or more
iSCSI nodes within a Network Entity. The iSCSI node is accessible
via one or more Network Portals. An iSCSI node is identified by
its iSCSI name. The separation of the iSCSI name from the
addresses used by and for the iSCSI node allows multiple iSCSI
nodes to use the same address and the same iSCSI node to use
multiple addresses.

iSCSI Target Name: The iSCSI Target Name specifies the worldwide
unique name of the target.

iSCSI Target Node: The iSCSI target node is the "target" device.
The word "target" has been appropriately qualified as either a port
or a device in the rest of the document when the context is
ambiguous. All unqualified usages of "target" refer to a target
port (or device), depending on the context.

iSCSI Task: An iSCSI task is an iSCSI request for which a response
is expected.

iSCSI Transfer Direction: The iSCSI transfer direction is defined
with regard to the initiator. Outbound or outgoing transfers are
transfers from the initiator to the target, while inbound or
incoming transfers are from the target to the initiator.

ISID: The ISID is the initiator part of the session identifier. It
is explicitly specified by the initiator during login.

I_T Nexus: According to [SAM2], the I_T nexus is a relationship
between a SCSI initiator port and a SCSI target port. For iSCSI,
this relationship is a session, defined as a relationship between
an iSCSI initiator's end of the session (SCSI initiator port) and
the iSCSI target's portal group. The I_T nexus can be identified
by the conjunction of the SCSI port names; that is, the I_T nexus
identifier is the tuple (iSCSI Initiator Name + ',i,' + ISID, iSCSI
Target Name + ',t,' + Target Portal Group Tag).

I_T_L Nexus: An I_T_L nexus is a SCSI concept and is defined as the
relationship between a SCSI initiator port, a SCSI target port, and
a Logical Unit (LU).

Network Entity: The Network Entity represents a device or gateway
that is accessible from the IP network. A Network Entity must have
one or more Network Portals, each of which can be used to gain
access to the IP network by some iSCSI nodes contained in that
Network Entity.

Network Portal: The Network Portal is a component of a Network
Entity that has a TCP/IP network address and that may be used by an
iSCSI node within that Network Entity for the connection(s) within
one of its iSCSI sessions. A Network Portal in an initiator is
identified by its IP address. A Network Portal in a target is
identified by its IP address and its listening TCP port.

Originator: In a negotiation or exchange, the originator is the
party that initiates the negotiation or exchange.

PDU (Protocol Data Unit): The initiator and target divide their
communications into messages. The term "iSCSI Protocol Data Unit"
(iSCSI PDU) is used for these messages.

Portal Groups: iSCSI supports multiple connections within the same
session; some implementations will have the ability to combine
connections in a session across multiple Network Portals. A portal
group defines a set of Network Portals within an iSCSI Network
Entity that collectively supports the capability of coordinating a
session with connections spanning these portals. Not all Network
Portals within a portal group need participate in every session
connected through that portal group. One or more portal groups may
provide access to an iSCSI node. Each Network Portal, as utilized
by a given iSCSI node, belongs to exactly one portal group within
that node.

Portal Group Tag: This 16-bit quantity identifies a portal group
within an iSCSI node. All Network Portals with the same Portal
Group Tag in the context of a given iSCSI node are in the same
portal group.

Recovery R2T: A recovery R2T is an R2T generated by a target upon
detecting the loss of one or more Data-Out PDUs through one of the
following means: a digest error, a sequence error, or a sequence
reception timeout. A recovery R2T carries the next unused R2TSN
but requests all or part of the data burst that an earlier R2T
(with a lower R2TSN) had already requested.

Responder: In a negotiation or exchange, the responder is the party
that responds to the originator of the negotiation or exchange.

SCSI Device: This is the SAM-2 term for an entity that contains one
or more SCSI ports that are connected to a service delivery
subsystem and supports a SCSI application protocol. For example, a
SCSI initiator device contains one or more SCSI initiator ports and
zero or more application clients. A target device contains one or
more SCSI target ports and one or more device servers and
associated LUs. For iSCSI, the SCSI device is the component within
an iSCSI node that provides the SCSI functionality. As such, there
can be at most one SCSI device within a given iSCSI node. Access
to the SCSI device can only be achieved in an iSCSI Normal
operational session. The SCSI device name is defined to be the
iSCSI name of the node.

Session: The group of TCP connections that link an initiator with a
target form a session (loosely equivalent to a SCSI I_T nexus).
TCP connections can be added and removed from a session. Across
all connections within a session, an initiator sees one and the
same target.

SCSI Port: This is the SAM-2 term for an entity in a SCSI device
that provides the SCSI functionality to interface with a service
delivery subsystem. For iSCSI, the definitions of the SCSI
initiator port and the SCSI target port are different.

SCSI Initiator Port: This maps to the endpoint of an iSCSI Normal
operational session. An iSCSI Normal operational session is
negotiated through the login process between an iSCSI initiator
node and an iSCSI target node. At successful completion of this
process, a SCSI initiator port is created within the SCSI initiator
device. The SCSI initiator port name and SCSI initiator port
identifier are both defined to be the iSCSI Initiator Name together
with (a) a label that identifies it as an initiator port
name/identifier and (b) the ISID portion of the session identifier.

SCSI Port Name: This is a name consisting of UTF-8 [RFC3629]
encoding of Unicode [UNICODE] characters and includes the iSCSI
name + 'i' or 't' + ISID or Target Portal Group Tag.

SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the aggregate
data length of the data that the SCSI layer logically "presents" to
the iSCSI layer for a Data-In or Data-Out transfer in the context
of a SCSI task. For a bidirectional task, there are two SPDTL
values -- one for Data-In and one for Data-Out. Note that the
notion of "presenting" includes immediate data per the data
transfer model in [SAM2] and excludes overlapping data transfers,
if any, requested by the SCSI layer.

SCSI Target Port: This maps to an iSCSI target portal group.

SCSI Target Port Name and SCSI Target Port Identifier: These are
both defined to be the iSCSI Target Name together with (a) a label
that identifies it as a target port name/identifier and (b) the
Target Portal Group Tag.

SSID (Session ID): A session between an iSCSI initiator and an
iSCSI target is defined by a session ID that is a tuple composed of
an initiator part (ISID) and a target part (Target Portal Group
Tag). The ISID is explicitly specified by the initiator at session
establishment. The Target Portal Group Tag is implied by the
initiator through the selection of the TCP endpoint at connection
establishment. The TargetPortalGroupTag key must also be returned
by the target as a confirmation during connection establishment.

T10: T10 is a technical committee within INCITS that develops
standards and technical reports on I/O interfaces, particularly the
series of SCSI (Small Computer System Interface) standards. See
<http://www.t10.org>.

T11: T11 is a technical committee within INCITS responsible for
standards development in the areas of Intelligent Peripheral
Interface (IPI), High-Performance Parallel Interface (HIPPI), and
Fibre Channel (FC). See <http://www.t11.org>.

Target Portal Group Tag: This is a numerical identifier (16-bit)
for an iSCSI target portal group.

Target Transfer Tag (TTT): The TTT is an iSCSI protocol field used
in a few iSCSI PDUs (e.g., R2T, NOP-In) that is always sent from
the target to the initiator first and then quoted as a reference in
initiator-sent PDUs back to the target relating to the same
task/exchange. Therefore, the TTT effectively acts as an opaque
handle to an existing task/exchange to help the target associate
the incoming PDUs from the initiator to the proper execution
context.

Third-party: This term is used in this document as a qualifier to
nexus objects (I_T or I_T_L) and iSCSI sessions, to indicate that
these objects and sessions reap the side effects of actions that
take place in the context of a separate iSCSI session. One example
of a third-party session is an iSCSI session discovering that its
I_T_L nexus to a LU got reset due to a LU reset operation
orchestrated via a separate I_T nexus.

TSIH (Target Session Identifying Handle): This is a target-assigned
tag for a session with a specific named initiator. The target
generates it during session establishment. Other than defining it
as a 16-bit binary string, its internal format and content are not
defined by this protocol but for the value with all bits set to 0
that is reserved and used by the initiator to indicate a new
session. It is given to the target during additional connection
establishment for the same session.

Added text in Section 9.3.3 noting that OCSP is now allowed for
checking certificates used with IPsec in addition to the use
of CRLs.

Added text in Section 9.3.1 specifying that extended sequence
numbers (ESNs) are now required for ESPv2 (part of IPsec v2).

2.4. Conventions

In examples, "I->" and "T->" show iSCSI PDUs sent by the initiator
and target, respectively.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

3. UML Conventions

3.1. UML Conventions Overview

The SCSI Architecture Model (SAM) uses class diagrams and object
diagrams with notation that is based on the Unified Modeling Language
[UML]. Therefore, this document also uses UML to model the
relationships for SCSI and iSCSI objects.

A treatise on the graphical notation used in UML is beyond the scope
of this document. However, given the use of ASCII drawing for UML
static class diagrams, a description of the notational conventions
used in this document is included in the remainder of this section.

3.2. Multiplicity Notion

Not specified The number of instances of an attribute is not
specified.
1 One instance of the class or attribute exists.
0..* Zero or more instances of the class or attribute
exist.
1..* One or more instances of the class or attribute
exist.
0..1 Zero or one instance of the class or attribute
exists.
n..m n to m instances of the class or attribute exist
(e.g., 2..8).

The preceding diagram is an example where Class A knows about
Class B (i.e., read as "Class A association_name Class B") and
Class B knows about Class A (i.e., read as "Class B
association_name Class A"). The use of association_name is
optional. The multiplicity notation (1..* and 0..1) indicates the
number of instances of the object.

3.5. Class Diagram Notation for Aggregations

The preceding diagram is an example where Class whole is an
aggregate that contains Class part and where Class part may
continue to exist even if Class whole is removed (i.e., read as
"the whole contains the part").

The preceding diagram is an example where Class whole is an
aggregate that contains Class part where Class part only belongs
to one Class whole, and the Class part does not continue to exist
if the Class whole is removed (i.e., read as "the whole contains
the part").

+-------------+
| |
+-------------+
| |
+ =(a)= +
| |

The preceding diagram is an example where there is a constraint
between the associations, where the (a) footnote describes the
constraint.

3.6. Class Diagram Notation for Generalizations

The preceding diagram is an example where the subclass is a kind
of superclass. A subclass shares all the attributes and
operations of the superclass (i.e., the subclass inherits from the
superclass).

4. Overview

4.1. SCSI Concepts

The SCSI Architecture Model - 2 [SAM2] describes in detail the
architecture of the SCSI family of I/O protocols. This section
provides a brief background of the SCSI architecture and is intended
to familiarize readers with its terminology.

At the highest level, SCSI is a family of interfaces for requesting
services from I/O devices, including hard drives, tape drives, CD and
DVD drives, printers, and scanners. In SCSI terminology, an
individual I/O device is called a "logical unit" (LU).

SCSI is a client-server architecture. Clients of a SCSI interface
are called "initiators". Initiators issue SCSI "commands" to request
services from components -- LUs of a server known as a "target". The
"device server" on the LU accepts SCSI commands and processes them.

A "SCSI transport" maps the client-server SCSI protocol to a specific
interconnect. The initiator is one endpoint of a SCSI transport.
The "target" is the other endpoint. A target can contain multiple
LUs. Each LU has an address within a target called a Logical Unit
Number (LUN).

A SCSI task is a SCSI command or possibly a linked set of SCSI
commands. Some LUs support multiple pending (queued) tasks, but the
queue of tasks is managed by the LU. The target uses an initiator-
provided "task tag" to distinguish between tasks. Only one command
in a task can be outstanding at any given time.

Each SCSI command results in an optional data phase and a required
response phase. In the data phase, information can travel from the
initiator to the target (e.g., write), from the target to the
initiator (e.g., read), or in both directions. In the response
phase, the target returns the final status of the operation,
including any errors.

Command Descriptor Blocks (CDBs) are the data structures used to
contain the command parameters that an initiator sends to a target.
The CDB content and structure are defined by [SAM2] and device-type
specific SCSI standards.

4.2. iSCSI Concepts and Functional Overview

The iSCSI protocol is a mapping of the SCSI command, event, and task
management model (see [SAM2]) over the TCP protocol. SCSI commands
are carried by iSCSI requests, and SCSI responses and status are
carried by iSCSI responses. iSCSI also uses the request-response
mechanism for iSCSI protocol mechanisms.

For the remainder of this document, the terms "initiator" and
"target" refer to "iSCSI initiator node" and "iSCSI target node",
respectively (see iSCSI), unless otherwise qualified.

As its title suggests, Section 4 presents an overview of the iSCSI
concepts, and later sections in the rest of the specification contain
the normative requirements -- in many cases covering the same
concepts discussed in Section 4. Such normative requirements text
overrides the overview text in Section 4 if there is a disagreement
between the two.

In keeping with similar protocols, the initiator and target divide
their communications into messages. This document uses the term
"iSCSI Protocol Data Unit" (iSCSI PDU) for these messages.

For performance reasons, iSCSI allows a "phase-collapse". A command
and its associated data may be shipped together from initiator to
target, and data and responses may be shipped together from targets.

The iSCSI transfer direction is defined with respect to the
initiator. Outbound or outgoing transfers are transfers from an
initiator to a target, while inbound or incoming transfers are from a
target to an initiator.

An iSCSI task is an iSCSI request for which a response is expected.

In this document, "iSCSI request", "iSCSI command", request, or
(unqualified) command have the same meaning. Also, unless otherwise
specified, status, response, or numbered response have the same
meaning.

4.2.1. Layers and Sessions

The following conceptual layering model is used to specify initiator
and target actions and the way in which they relate to transmitted
and received Protocol Data Units:

the iSCSI layer that builds/receives iSCSI PDUs and
relays/receives them to/from one or more TCP connections; the
group of connections form an initiator-target "session".

Communication between the initiator and target occurs over one or
more TCP connections. The TCP connections carry control messages,
SCSI commands, parameters, and data within iSCSI Protocol Data Units
(iSCSI PDUs). The group of TCP connections that link an initiator
with a target form a session (equivalent to a SCSI I_T nexus; see
Section 4.4.2). A session is defined by a session ID that is
composed of an initiator part and a target part. TCP connections can
be added and removed from a session. Each connection within a
session is identified by a connection ID (CID).

Across all connections within a session, an initiator sees one
"target image". All target-identifying elements, such as a LUN, are
the same. A target also sees one "initiator image" across all
connections within a session. Initiator-identifying elements, such
as the Initiator Task Tag, are global across the session, regardless
of the connection on which they are sent or received.

iSCSI targets and initiators MUST support at least one TCP connection
and MAY support several connections in a session. For error recovery
purposes, targets and initiators that support a single active
connection in a session SHOULD support two connections during
recovery.

4.2.2. Ordering and iSCSI Numbering

Command numbering is session-wide and is used for ordered command
delivery over multiple connections. It can also be used as a
mechanism for command flow control over a session.

Status numbering is per connection and is used to enable missing
status detection and recovery in the presence of transient or
permanent communication errors.

Data sequencing is per command or part of a command (R2T-triggered
sequence) and is used to detect missing data and/or R2T PDUs due to
header digest errors.

Typically, fields in the iSCSI PDUs communicate the sequence numbers
between the initiator and target. During periods when traffic on a
connection is unidirectional, iSCSI NOP-Out/NOP-In PDUs may be
utilized to synchronize the command and status ordering counters of
the target and initiator.

The iSCSI session abstraction is equivalent to the SCSI I_T nexus,
and the iSCSI session provides an ordered command delivery from the
SCSI initiator to the SCSI target. For detailed design
considerations that led to the iSCSI session model as it is defined
here and how it relates the SCSI command ordering features defined in
SCSI specifications to the iSCSI concepts, see [RFC3783].

4.2.2.1. Command Numbering and Acknowledging

iSCSI performs ordered command delivery within a session. All
commands (initiator-to-target PDUs) in transit from the initiator to
the target are numbered.

iSCSI considers a task to be instantiated on the target in response
to every request issued by the initiator. A set of task management
operations, including abort and reassign (see Section 11.5), may be
performed on an iSCSI task; however, an abort operation cannot be
performed on a task management operation, and usage of reassign
operations has certain constraints. See Section 11.5.1 for details.

Some iSCSI tasks are SCSI tasks, and many SCSI activities are related
to a SCSI task ([SAM2]). In all cases, the task is identified by the
Initiator Task Tag for the life of the task.

The command number is carried by the iSCSI PDU as the CmdSN (command
sequence number). The numbering is session-wide. Outgoing iSCSI
PDUs carry this number. The iSCSI initiator allocates CmdSNs with a
32-bit unsigned counter (modulo 2**32). Comparisons and arithmetic
on CmdSNs use Serial Number Arithmetic as defined in [RFC1982] where
SERIAL_BITS = 32.

Commands meant for immediate delivery are marked with an immediate
delivery flag; they MUST also carry the current CmdSN. The CmdSN
MUST NOT advance after a command marked for immediate delivery is
sent.

Command numbering starts with the first Login Request on the first
connection of a session (the leading login on the leading
connection), and the CmdSN MUST be incremented by 1 in a Serial
Number Arithmetic sense, as defined in [RFC1982], for every
non-immediate command issued afterwards.

If immediate delivery is used with task management commands, these
commands may reach the target before the tasks on which they are
supposed to act. However, their CmdSN serves as a marker of their
position in the stream of commands. The initiator and target MUST
ensure that the SCSI task management functions specified in [SAM2]
act in accordance with the [SAM2] specification. For example, both
commands and responses appear as if delivered in order. Whenever the
CmdSN for an outgoing PDU is not specified by an explicit rule, the
CmdSN will carry the current value of the local CmdSN variable (see
later in this section).

The means by which an implementation decides to mark a PDU for
immediate delivery or by which iSCSI decides by itself to mark a PDU
for immediate delivery are beyond the scope of this document.

The number of commands used for immediate delivery is not limited,
and their delivery to execution is not acknowledged through the
numbering scheme. An iSCSI target MAY reject immediate commands,
e.g., due to lack of resources to accommodate additional commands.
An iSCSI target MUST be able to handle at least one immediate task
management command and one immediate non-task-management iSCSI
command per connection at any time.

In this document, delivery for execution means delivery to the SCSI
execution engine or an iSCSI protocol-specific execution engine
(e.g., for Text Requests with public or private extension keys
involving an execution component). With the exception of the
commands marked for immediate delivery, the iSCSI target layer MUST
deliver the commands for execution in the order specified by the
CmdSN. Commands marked for immediate delivery may be delivered by
the iSCSI target layer for execution as soon as detected. iSCSI may
avoid delivering some commands to the SCSI target layer if required
by a prior SCSI or iSCSI action (e.g., a CLEAR TASK SET task
management request received before all the commands on which it was
supposed to act).

On any connection, the iSCSI initiator MUST send the commands in
increasing order of CmdSN, except for commands that are retransmitted
due to digest error recovery and connection recovery.

For the numbering mechanism, the initiator and target maintain the
following three variables for each session:

CmdSN: the current command sequence number, advanced by 1 on
each command shipped except for commands marked for immediate
delivery as discussed above. The CmdSN always contains the
number to be assigned to the next command PDU.

ExpCmdSN: the next expected command by the target. The target
acknowledges all commands up to, but not including, this number.
The initiator treats all commands with a CmdSN less than the
ExpCmdSN as acknowledged. The target iSCSI layer sets the
ExpCmdSN to the largest non-immediate CmdSN that it can deliver
for execution "plus 1" per [RFC1982]. There MUST NOT be any
holes in the acknowledged CmdSN sequence.

MaxCmdSN: the maximum number to be shipped. The queuing
capacity of the receiving iSCSI layer is
MaxCmdSN - ExpCmdSN + 1.

The initiator's ExpCmdSN and MaxCmdSN are derived from target-to-
initiator PDU fields. Comparisons and arithmetic on the ExpCmdSN and
MaxCmdSN MUST use Serial Number Arithmetic as defined in [RFC1982]
where SERIAL_BITS = 32.

The target MUST NOT transmit a MaxCmdSN that is less than
ExpCmdSN - 1. For non-immediate commands, the CmdSN field can take
any value from the ExpCmdSN to the MaxCmdSN inclusive. The target
MUST silently ignore any non-immediate command outside of this range
or non-immediate duplicates within the range. The CmdSN carried by
immediate commands may lie outside the ExpCmdSN-to-MaxCmdSN range.
For example, if the initiator has previously sent a non-immediate
command carrying the CmdSN equal to the MaxCmdSN, the target window
is closed. For group task management commands issued as immediate
commands, the CmdSN indicates the scope of the group action (e.g., an
ABORT TASK SET indicates which commands are to be aborted).

MaxCmdSN and ExpCmdSN fields are processed by the initiator as
follows:

If the PDU MaxCmdSN is less than the PDU ExpCmdSN - 1 (in a
Serial Number Arithmetic sense), they are both ignored.

If the PDU MaxCmdSN is greater than the local MaxCmdSN (in a
Serial Number Arithmetic sense), it updates the local MaxCmdSN;
otherwise, it is ignored.

If the PDU ExpCmdSN is greater than the local ExpCmdSN (in a
Serial Number Arithmetic sense), it updates the local ExpCmdSN;
otherwise, it is ignored.

This sequence is required because updates may arrive out of order
(e.g., the updates are sent on different TCP connections).

iSCSI initiators and targets MUST support the command numbering
scheme.

A numbered iSCSI request will not change its allocated CmdSN,
regardless of the number of times and circumstances in which it is
reissued (see Section 7.2.1). At the target, the CmdSN is only
relevant while the command has not created any state related to its
execution (execution state); afterwards, the CmdSN becomes
irrelevant. Testing for the execution state (represented by
identifying the Initiator Task Tag) MUST precede any other action at
the target. If no execution state is found, it is followed by
ordering and delivery. If an execution state is found, it is
followed by delivery if it has not already been delivered.

If an initiator issues a command retry for a command with CmdSN R on
a connection when the session CmdSN value is Q, it MUST NOT advance
the CmdSN past R + 2**31 - 1 unless

- the connection is no longer operational (i.e., it has returned
to the FREE state; see Section 8.1.3),

the connection has been reinstated (see Section 6.3.4), or

a non-immediate command with a CmdSN equal to or greater than Q
was issued subsequent to the command retry on the same
connection and the reception of that command is acknowledged by
the target (see Section 10.4).

A target command response or Data-In PDU with status MUST NOT precede
the command acknowledgment. However, the acknowledgment MAY be
included in the response or the Data-In PDU.

4.2.2.2. Response/Status Numbering and Acknowledging

Responses in transit from the target to the initiator are numbered.
The StatSN (status sequence number) is used for this purpose. The
StatSN is a counter maintained per connection. The ExpStatSN is used
by the initiator to acknowledge status. The status sequence number
space is 32-bit unsigned integers, and the arithmetic operations are
the regular mod(2**32) arithmetic.

Status numbering starts with the Login Response to the first Login
Request of the connection. The Login Response includes an initial
value for status numbering (any initial value is valid).

To enable command recovery, the target MAY maintain enough state
information for data and status recovery after a connection failure.
A target doing so can safely discard all of the state information
maintained for recovery of a command after the delivery of the status
for the command (numbered StatSN) is acknowledged through the
ExpStatSN.

A large absolute difference between the StatSN and the ExpStatSN may
indicate a failed connection. Initiators MUST undertake recovery
actions if the difference is greater than an implementation-defined
constant that MUST NOT exceed 2**31 - 1.

Initiators and targets MUST support the response-numbering scheme.

4.2.2.3. Response Ordering

4.2.2.3.1. Need for Response Ordering

Whenever an iSCSI session is composed of multiple connections, the
Response PDUs (task responses or TMF Responses) originating in the
target SCSI layer are distributed onto the multiple connections by
the target iSCSI layer according to iSCSI connection allegiance
rules. This process generally may not preserve the ordering of the
responses by the time they are delivered to the initiator SCSI layer.

Since ordering is not expected across SCSI Response PDUs anyway, this
approach works fine in the general case. However, to address the
special cases where some ordering is desired by the SCSI layer, we
introduce the notion of a "Response Fence": a Response Fence is
logically the attribute/property of a SCSI response message handed
off to a target iSCSI layer that indicates that there are special
SCSI-level ordering considerations associated with this particular
response message. Whenever a Response Fence is set or required on a

SCSI response message, we define the semantics in Section 4.2.2.3.2
with respect to the target iSCSI layer's handling of such SCSI
response messages.

Whenever the Response Fence behavior is required for a SCSI response
message, the target iSCSI layer MUST ensure that the following
conditions are met in delivering the response message to the
initiator iSCSI layer:

A response with a Response Fence MUST be delivered
chronologically after all the "preceding" responses on the I_T_L
nexus, if the preceding responses are delivered at all, to the
initiator iSCSI layer.

A response with a Response Fence MUST be delivered
chronologically prior to all the "following" responses on the
I_T_L nexus.

The notions of "preceding" and "following" refer to the order of
handoff of a response message from the target SCSI protocol layer to
the target iSCSI layer.

4.2.2.3.3. iSCSI Semantics with the Interface Model

Whenever the TaskReporting key (Section 13.23) is negotiated to
ResponseFence or FastAbort for an iSCSI session and the Response
Fence behavior is required for a SCSI response message, the target
iSCSI layer MUST perform the actions described in this section for
that session.

takes note of the last-sent and unacknowledged StatSN on each
of the connections in the iSCSI session, and waits for an
acknowledgment (NOP-In PDUs MAY be used to solicit
acknowledgments as needed in order to accelerate this process)
of each such StatSN to clear the fence. The SCSI Response PDU
requiring the Response Fence behavior MUST NOT be sent to the
initiator before acknowledgments are received for each of the
unacknowledged StatSNs.

c) The target iSCSI layer must wait for an acknowledgment of the

SCSI Response PDU that carried the SCSI response requiring the
Response Fence behavior. The fence MUST be considered cleared
only after receiving the acknowledgment.

d) All further status processing for the LU is resumed only after

clearing the fence. If any new responses for the I_T_L nexus
are received from the SCSI layer before the fence is cleared,
those Response PDUs MUST be held and queued at the iSCSI layer
until the fence is cleared.

4.2.2.3.4. Current List of Fenced Response Use Cases

This section lists the situations in which fenced response behavior
is REQUIRED in iSCSI target implementations. Note that the following
list is an exhaustive enumeration as currently identified -- it is
expected that as SCSI protocol specifications evolve, the
specifications will enumerate when response fencing is required on a
case-by-case basis.

Whenever the TaskReporting key (Section 13.23) is negotiated to
ResponseFence or FastAbort for an iSCSI session, the target iSCSI
layer MUST assume that the Response Fence is required for the
following SCSI completion messages:

Due to the absence of ACA-related fencing requirements in
[RFC3720], initiator implementations SHOULD NOT use ACA on
multi-connection iSCSI sessions with targets complying only with
[RFC3720]. This can be determined via TaskReporting key
(Section 13.23) negotiation -- when the negotiation results in
either "RFC3720" or "NotUnderstood".

Initiators that want to employ ACA on multi-connection iSCSI
sessions SHOULD first assess response-fencing behavior via
negotiating for the "ResponseFence" or "FastAbort" value for the
TaskReporting (Section 13.23) key.

4.2.2.4. Data Sequencing

Data and R2T PDUs transferred as part of some command execution MUST
be sequenced. The DataSN field is used for data sequencing. For
input (read) data PDUs, the DataSN starts with 0 for the first data
PDU of an input command and advances by 1 for each subsequent data
PDU. For output data PDUs, the DataSN starts with 0 for the first
data PDU of a sequence (the initial unsolicited sequence or any data
PDU sequence issued to satisfy an R2T) and advances by 1 for each
subsequent data PDU. R2Ts are also sequenced per command. For
example, the first R2T has an R2TSN of 0 and advances by 1 for each
subsequent R2T. For bidirectional commands, the target uses the
DataSN/R2TSN to sequence Data-In and R2T PDUs in one continuous
sequence (undifferentiated). Unlike command and status, data PDUs
and R2Ts are not acknowledged by a field in regular outgoing PDUs.
Data-In PDUs can be acknowledged on demand by a special form of the
SNACK PDU. Data and R2T PDUs are implicitly acknowledged by status
for the command. The DataSN/R2TSN field enables the initiator to
detect missing data or R2T PDUs.

For any read or bidirectional command, a target MUST issue less than
2**32 combined R2T and Data-In PDUs. Any output data sequence MUST
contain less than 2**32 Data-Out PDUs.

4.2.3.2. Notion of Affected Tasks

This section defines the notion of "affected tasks" in multi-task
abort scenarios. Scope definitions in this section apply to both the
standard multi-task abort semantics (Section 4.2.3.3) and the
FastAbort multi-task abort semantics behavior (Section 4.2.3.4).

ABORT TASK SET: All outstanding tasks for the I_T_L nexus identified

by the LUN field in the ABORT TASK SET TMF Request PDU.

CLEAR TASK SET: All outstanding tasks in the task set for the LU

identified by the LUN field in the CLEAR TASK SET TMF Request PDU.
See [SPC3] for the definition of a "task set".

LOGICAL UNIT RESET: All outstanding tasks from all initiators for the

LU identified by the LUN field in the LOGICAL UNIT RESET
Request PDU.

TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks from all

initiators across all LUs to which the TMF-issuing session has
access on the SCSI target device hosting the iSCSI session.

Usage: An "ABORT TASK SET TMF Request PDU" in the preceding text is

an iSCSI TMF Request PDU with the "Function" field set to "ABORT
TASK SET" as defined in Section 11.5. Similar usage is employed
for other scope descriptions.

4.2.3.3. Standard Multi-Task Abort Semantics

All iSCSI implementations MUST support the protocol behavior defined
in this section as the default behavior. The execution of ABORT TASK
SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET, and
TARGET COLD RESET TMF Requests consists of the following sequence of
actions in the specified order on the specified party.

The initiator iSCSI layer:

a) MUST continue to respond to each TTT received for the affected

tasks.

b) SHOULD process any responses received for affected tasks in the

normal fashion. This is acceptable because the responses are
guaranteed to have been sent prior to the TMF Response.

c) SHOULD receive the TMF Response concluding all the tasks in the

set of affected tasks, unless the initiator has done something
(e.g., LU reset, connection drop) that may prevent the TMF
Response from being sent or received. The initiator MUST thus
conclude all affected tasks as part of this step in either case
and MUST discard any TMF Response received after the affected
tasks are concluded.

The target iSCSI layer:

a) MUST wait for responses on currently valid Target Transfer Tags

of the affected tasks from the issuing initiator. MAY wait for
responses on currently valid Target Transfer Tags of the
affected tasks from third-party initiators.

b) MUST wait (concurrent with the wait in Step a) for all commands

of the affected tasks to be received based on the CmdSN
ordering. SHOULD NOT wait for new commands on third-party
affected sessions -- only the instantiated tasks have to be
considered for the purpose of determining the affected tasks.
However, in the case of target-scoped requests (i.e., TARGET
WARM RESET and TARGET COLD RESET), all of the commands that are
not yet received on the issuing session in the command stream
can be considered to have been received with no command waiting
period -- i.e., the entire CmdSN space up to the CmdSN of the
task management function can be "plugged".

c) MUST propagate the TMF Request to, and receive the response

from, the target SCSI layer.

d) MUST provide the Response Fence behavior for the TMF Response

on the issuing session as specified in Section 4.2.2.3.2.

e) MUST provide the Response Fence behavior on the first post-TMF

Response on third-party sessions as specified in
Section 4.2.2.3.3. If some tasks originate from non-iSCSI
I_T_L nexuses, then the means by which the target ensures that
all affected tasks have returned their status to the initiator
are defined by the specific non-iSCSI transport protocol(s).

Technically, the TMF servicing is complete in Step d). Data
transfers corresponding to terminated tasks may, however, still be in
progress on third-party iSCSI sessions even at the end of Step e).
The TMF Response MUST NOT be sent by the target iSCSI layer before
the end of Step d) and MAY be sent at the end of Step d) despite
these outstanding data transfers until after Step e).

4.2.3.4. FastAbort Multi-Task Abort Semantics

Protocol behavior defined in this section SHOULD be implemented by
all iSCSI implementations complying with this document, noting that
some steps below may not be compatible with [RFC3720] semantics.
However, protocol behavior defined in this section MUST be exhibited
by iSCSI implementations on an iSCSI session when they negotiate the
TaskReporting (Section 13.23) key to "FastAbort" on that session.
The execution of ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET,
TARGET WARM RESET, and TARGET COLD RESET TMF Requests consists of the
following sequence of actions in the specified order on the specified
party.

The initiator iSCSI layer:

a) MUST NOT send any more Data-Out PDUs for affected tasks on the

issuing connection of the issuing iSCSI session once the TMF is
sent to the target.

b) SHOULD process any responses received for affected tasks in the

normal fashion. This is acceptable because the responses are
guaranteed to have been sent prior to the TMF Response.

c) MUST respond to each Async Message PDU with a Task Termination

AsyncEvent (5) as defined in Section 11.9.

d) MUST treat the TMF Response as terminating all affected tasks

for which responses have not been received and MUST discard any
responses for affected tasks received after the TMF Response is
passed to the SCSI layer (although the semantics defined in
this section ensure that such an out-of-order scenario will
never happen with a compliant target implementation).

The target iSCSI layer:

a) MUST wait for all commands of the affected tasks to be received

based on the CmdSN ordering on the issuing session. SHOULD NOT
wait for new commands on third-party affected sessions -- only
the instantiated tasks have to be considered for the purpose of
determining the affected tasks. In the case of target-scoped
requests (i.e., TARGET WARM RESET and TARGET COLD RESET), all
the commands that are not yet received on the issuing session
in the command stream can be considered to have been received
with no command waiting period -- i.e., the entire CmdSN space
up to the CmdSN of the task management function can be
"plugged".

each connection of each third-party session to which at
least one affected task is allegiant if
TaskReporting=FastAbort is operational on that third-party
session, and

each connection except the issuing connection of the issuing
session that has at least one allegiant affected task.

If there are multiple affected LUs (say, due to a target
reset), then one Async Message PDU MUST be sent for each
such LU on each connection that has at least one allegiant
affected task. The LUN field in the Asynchronous Message
PDU MUST be set to match the LUN for each such LU.

e) MUST address the Response Fence flag on the TMF Response on the

issuing session as defined in Section 4.2.2.3.3.

f) MUST address the Response Fence flag on the first post-TMF

Response on third-party sessions as defined in
Section 4.2.2.3.3. If some tasks originate from non-iSCSI
I_T_L nexuses, then the means by which the target ensures that
all affected tasks have returned their status to the initiator
are defined by the specific non-iSCSI transport protocol(s).

g) MUST free up the affected TTTs (and STags for iSER, if

applicable) and the corresponding buffers, if any, once it

receives each associated NOP-Out acknowledgment that the

initiator generated in response to each Async Message.

Technically, the TMF servicing is complete in Step e). Data
transfers corresponding to terminated tasks may, however, still be in
progress even at the end of Step f). A TMF Response MUST NOT be sent
by the target iSCSI layer before the end of Step e) and MAY be sent
at the end of Step e) despite these outstanding Data transfers until
Step g). Step g) specifies an event to free up any such resources
that may have been reserved to support outstanding data transfers.

4.2.3.5. Affected Tasks Shared across Standard and FastAbort Sessions

If an iSCSI target implementation is capable of supporting
TaskReporting=FastAbort functionality (Section 13.23), it may end up
in a situation where some sessions have TaskReporting=RFC3720
operational (RFC 3720 sessions) while some other sessions have
TaskReporting=FastAbort operational (FastAbort sessions) even while
accessing a shared set of affected tasks (Section 4.2.3.2). If the
issuing session is an RFC 3720 session, the iSCSI target
implementation is FastAbort-capable, and the third-party affected
session is a FastAbort session, the following behavior SHOULD be
exhibited by the iSCSI target layer:

a) Between Steps c) and d) of the target behavior in

Section 4.2.3.3, send an Asynchronous Message PDU with
AsyncEvent=5 (Section 11.9) on each connection of each third-
party session to which at least one affected task is allegiant.
If there are multiple affected LUs, then send one Async Message
PDU for each such LU on each connection that has at least one
allegiant affected task. When sent, the LUN field in the
Asynchronous Message PDU MUST be set to match the LUN for each
such LU.

b) After Step e) of the target behavior in Section 4.2.3.3, free

up the affected TTTs (and STags for iSER, if applicable) and
the corresponding buffers, if any, once each associated NOP-Out
acknowledgment is received that the third-party initiator
generated in response to each Async Message sent in Step a).

If the issuing session is a FastAbort session, the iSCSI target
implementation is FastAbort-capable, and the third-party affected
session is an RFC 3720 session, the iSCSI target layer MUST NOT send
Asynchronous Message PDUs on the third-party session to prompt the
FastAbort behavior.

If the third-party affected session is a FastAbort session and the
issuing session is a FastAbort session, the initiator in the third-
party role MUST respond to each Async Message PDU with AsyncEvent=5
as defined in Section 11.9. Note that an initiator MAY thus receive
these Async Messages on a third-party affected session even if the
session is a single-connection session.

4.2.3.6. Rationale behind the FastAbort Semantics

There are fundamentally three basic objectives behind the semantics
specified in Sections 4.2.3.3 and 4.2.3.4.

a) Maintaining an ordered command flow I_T nexus abstraction to

the target SCSI layer even with multi-connection sessions.

Target iSCSI processing of a TMF Request must maintain the
single flow illusion. The target behavior in Step b) of
Section 4.2.3.3 and the target behavior in Step a) of
Section 4.2.3.4 correspond to this objective.

b) Maintaining a single ordered response flow I_T nexus

abstraction to the initiator SCSI layer even with multi-
connection sessions when one response (i.e., TMF Response)
could imply the status of other unfinished tasks from the
initiator's perspective.

The target must ensure that the initiator does not see "old"
task responses (that were placed on the wire chronologically
earlier than the TMF Response) after seeing the TMF Response.
The target behavior in Step d) of Section 4.2.3.3 and the
target behavior in Step e) of Section 4.2.3.4 correspond to
this objective.

Whenever the result of a TMF action is visible across
multiple I_T_L nexuses, [SAM2] requires the SCSI device
server to trigger a UA on each of the other I_T_L nexuses.
Once an initiator is notified of such a UA, the application
client on the receiving initiator is required to clear its
task state (Clause 5.5 of [SAM2]) for the affected tasks. It
would thus be inappropriate to deliver a SCSI Response for a
task after the task state is cleared on the initiator, i.e.,
after the UA is notified. The UA notification contained in
the first SCSI Response PDU on each affected third-party
I_T_L nexus after the TMF action thus MUST NOT pass the
affected task responses on any of the iSCSI sessions
accessing the LU. The target behavior in Step e) of
Section 4.2.3.3 and the target behavior in Step f) of
Section 4.2.3.4 correspond to this objective.

c) Draining all active TTTs corresponding to affected tasks in a

deterministic fashion.

Data-Out PDUs with stale TTTs arriving after the tasks are
terminated can create a buffer management problem even for
traditional iSCSI implementations and is fatal for the
connection for iSCSI/iSER implementations. Either the
termination of affected tasks should be postponed until the
TTTs are retired (as in Step a) of Section 4.2.3.3), or the
TTTs and the buffers should stay allocated beyond task
termination to be deterministically freed up later (as in
Steps c) and g) of Section 4.2.3.4).

The only other notable optimization is the plugging. If all tasks on
an I_T nexus will be aborted anyway (as with a target reset), there
is no need to wait to receive all commands to plug the CmdSN holes.
The target iSCSI layer can simply plug all missing CmdSN slots and
move on with TMF processing. The first objective (maintaining a
single ordered command flow) is still met with this optimization
because the target SCSI layer only sees ordered commands.

4.2.4. iSCSI Login

The purpose of the iSCSI login is to enable a TCP connection for
iSCSI use, authentication of the parties, negotiation of the
session's parameters, and marking of the connection as belonging to
an iSCSI session.

A session is used to identify to a target all the connections with a
given initiator that belong to the same I_T nexus. (For more details
on how a session relates to an I_T nexus, see Section 4.4.2.)

The targets listen on a well-known TCP port or other TCP port for
incoming connections. The initiator begins the login process by
connecting to one of these TCP ports.

As part of the login process, the initiator and target SHOULD
authenticate each other and MAY set a security association protocol
for the session. This can occur in many different ways and is
subject to negotiation; see Section 12.

To protect the TCP connection, an IPsec security association MAY be
established before the Login Request. For information on using IPsec
security for iSCSI, see Section 9, [RFC3723], and [RFC7146].

The iSCSI Login Phase is carried through Login Requests and
Responses. Once suitable authentication has occurred and operational
parameters have been set, the session transitions to the Full Feature
Phase and the initiator may start to send SCSI commands. The
security policy for whether and by what means a target chooses to
authorize an initiator is beyond the scope of this document. For a
more detailed description of the Login Phase, see Section 6.

The login PDU includes the ISID part of the session ID (SSID). The
target portal group that services the login is implied by the
selection of the connection endpoint. For a new session, the TSIH is
zero. As part of the response, the target generates a TSIH.

During session establishment, the target identifies the SCSI
initiator port (the "I" in the "I_T nexus") through the value pair
(InitiatorName, ISID). We describe InitiatorName later in this
section. Any persistent state (e.g., persistent reservations) on the
target that is associated with a SCSI initiator port is identified
based on this value pair. Any state associated with the SCSI target
port (the "T" in the "I_T nexus") is identified externally by the
TargetName and Target Portal Group Tag (see Section 4.4.1). The ISID
is subject to reuse restrictions because it is used to identify a
persistent state (see Section 4.4.3).

Before the Full Feature Phase is established, only Login Request and
Login Response PDUs are allowed. Login Requests and Responses MUST
be used exclusively during login. On any connection, the Login Phase
MUST immediately follow TCP connection establishment, and a
subsequent Login Phase MUST NOT occur before tearing down the
connection.

A target receiving any PDU except a Login Request before the Login
Phase is started MUST immediately terminate the connection on which
the PDU was received. Once the Login Phase has started, if the
target receives any PDU except a Login Request, it MUST send a Login
reject (with Status "invalid during login") and then disconnect. If
the initiator receives any PDU except a Login Response, it MUST
immediately terminate the connection.

4.2.5. iSCSI Full Feature Phase

Once the two sides successfully conclude the login on the first --
also called the leading -- connection in the session, the iSCSI
session is in the iSCSI Full Feature Phase. A connection is in the
Full Feature Phase if the session is in the Full Feature Phase and
the connection login has completed successfully. An iSCSI connection
is not in the Full Feature Phase when

a) it does not have an established transport connection, or

b) when it has a valid transport connection, but a successful

login was not performed or the connection is currently
logged out.

In a normal Full Feature Phase, the initiator may send SCSI commands
and data to the various LUs on the target by encapsulating them in
iSCSI PDUs that go over the established iSCSI session.

4.2.5.1. Command Connection Allegiance

For any iSCSI request issued over a TCP connection, the corresponding
response and/or other related PDU(s) MUST be sent over the same
connection. We call this "connection allegiance". If the original
connection fails before the command is completed, the connection
allegiance of the command may be explicitly reassigned to a different
transport connection as described in detail in Section 7.2.

Thus, if an initiator issues a read command, the target MUST send the
requested data, if any, followed by the status, to the initiator over
the same TCP connection that was used to deliver the SCSI command.
If an initiator issues a write command, the initiator MUST send the
data, if any, for that command over the same TCP connection that was
used to deliver the SCSI command. The target MUST return Ready To
Transfer (R2T), if any, and the status over the same TCP connection
that was used to deliver the SCSI command. Retransmission requests
(SNACK PDUs), and the data and status that they generate, MUST also
use the same connection.

However, consecutive commands that are part of a SCSI linked command-
chain task (see [SAM2]) MAY use different connections. Connection
allegiance is strictly per command and not per task. During the
iSCSI Full Feature Phase, the initiator and target MAY interleave
unrelated SCSI commands, their SCSI data, and responses over the
session.

4.2.5.2. Data Transfer Overview

Outgoing SCSI data (initiator-to-target user data or command
parameters) is sent as either solicited data or unsolicited data.
Solicited data are sent in response to R2T PDUs. Unsolicited data
can be sent as part of an iSCSI Command PDU ("immediate data") or in
separate iSCSI data PDUs.

Immediate data are assumed to originate at offset 0 in the initiator
SCSI write-buffer (outgoing data buffer). All other data PDUs have
the buffer offset set explicitly in the PDU header.

An initiator may send unsolicited data up to FirstBurstLength (see
Section 13.14) as immediate (up to the negotiated maximum PDU
length), in a separate PDU sequence, or both. All subsequent data
MUST be solicited. The maximum length of an individual data PDU or
the immediate-part of the first unsolicited burst MAY be negotiated
at login.

The maximum amount of unsolicited data that can be sent with a
command is negotiated at login through the FirstBurstLength (see
Section 13.14) key. A target MAY separately enable immediate data
(through the ImmediateData key) without enabling the more general
(separate data PDUs) form of unsolicited data (through the
InitialR2T key).

Unsolicited data for a write are meant to reduce the effect of
latency on throughput (no R2T is needed to start sending data). In
addition, immediate data is meant to reduce the protocol overhead
(both bandwidth and execution time).

An iSCSI initiator MAY choose not to send unsolicited data, only
immediate data or FirstBurstLength bytes of unsolicited data with a
command. If any non-immediate unsolicited data is sent, the total
unsolicited data MUST be either FirstBurstLength or all of the data,
if the total amount is less than the FirstBurstLength.

It is considered an error for an initiator to send unsolicited data
PDUs to a target that operates in R2T mode (only solicited data are
allowed). It is also an error for an initiator to send more
unsolicited data, whether immediate or as separate PDUs, than
FirstBurstLength.

An initiator MUST honor an R2T data request for a valid outstanding
command (i.e., carrying a valid Initiator Task Tag) and deliver all
the requested data, provided the command is supposed to deliver
outgoing data and the R2T specifies data within the command bounds.
The initiator action is unspecified for receiving an R2T request that
specifies data, all or in part, outside of the bounds of the command.

A target SHOULD NOT silently discard data and then request
retransmission through R2T. Initiators SHOULD NOT keep track of the
data transferred to or from the target (scoreboarding). SCSI targets
perform residual count calculation to check how much data was
actually transferred to or from the device by a command. This may
differ from the amount the initiator sent and/or received for reasons
such as retransmissions and errors. Read or bidirectional commands
implicitly solicit the transmission of the entire amount of data
covered by the command. SCSI data packets are matched to their
corresponding SCSI commands by using tags specified in the protocol.

In addition, iSCSI initiators and targets MUST enforce some ordering
rules. When unsolicited data is used, the order of the unsolicited
data on each connection MUST match the order in which the commands on
that connection are sent. Command and unsolicited data PDUs may be
interleaved on a single connection as long as the ordering
requirements of each are maintained (e.g., command N + 1 MAY be sent
before the unsolicited Data-Out PDUs for command N, but the
unsolicited Data-Out PDUs for command N MUST precede the unsolicited
Data-Out PDUs of command N + 1). A target that receives data out of
order MAY terminate the session.

4.2.5.3. Tags and Integrity Checks

Initiator tags for pending commands are unique initiator-wide for a
session. Target tags are not strictly specified by the protocol. It
is assumed that target tags are used by the target to tag (alone or
in combination with the LUN) the solicited data. Target tags are
generated by the target and "echoed" by the initiator.

These mechanisms are designed to accomplish efficient data delivery
along with a large degree of control over the data flow.

As the Initiator Task Tag is used to identify a task during its
execution, the iSCSI initiator and target MUST verify that all other
fields used in task-related PDUs have values that are consistent with
the values used at the task instantiation, based on the Initiator
Task Tag (e.g., the LUN used in an R2T PDU MUST be the same as the
one used in the SCSI Command PDU used to instantiate the task).
Using inconsistent field values is considered a protocol error.

4.2.5.4. SCSI Task Management during iSCSI Full Feature Phase

SCSI task management assumes that individual tasks and task groups
can be aborted based solely on the task tags (for individual tasks)
or the timing of the task management command (for task groups) and
that the task management action is executed synchronously -- i.e., no
message involving an aborted task will be seen by the SCSI initiator
after receiving the task management response. In iSCSI, initiators
and targets interact asynchronously over several connections. iSCSI
specifies the protocol mechanism and implementation requirements
needed to present a synchronous SCSI view while using an asynchronous
iSCSI infrastructure.

4.2.6. iSCSI Connection Termination

An iSCSI connection may be terminated via a transport connection
shutdown or a transport reset. A transport reset is assumed to be an
exceptional event.

Graceful TCP connection shutdowns are done by sending TCP FINs. A
graceful transport connection shutdown SHOULD only be initiated by
either party when the connection is not in the iSCSI Full Feature
Phase. A target MAY terminate a Full Feature Phase connection on
internal exception events, but it SHOULD announce the fact through an
Asynchronous Message PDU. Connection termination with outstanding
commands may require recovery actions.

If a connection is terminated while in the Full Feature Phase,
connection cleanup (see Section 7.14) is required prior to recovery.
By doing connection cleanup before starting recovery, the initiator
and target will avoid receiving stale PDUs after recovery.

4.2.7. iSCSI Names

Both targets and initiators require names for the purpose of
identification. In addition, names enable iSCSI storage resources to
be managed, regardless of location (address). An iSCSI Node Name is
also the SCSI device name contained in the iSCSI node. The iSCSI
name of a SCSI device is the principal object used in authentication
of targets to initiators and initiators to targets. This name is
also used to identify and manage iSCSI storage resources.

iSCSI names must be unique within the operation domain of the end
user. However, because the operation domain of an IP network is
potentially worldwide, the iSCSI name formats are architected to be
worldwide unique. To assist naming authorities in the construction
of worldwide unique names, iSCSI provides three name formats for
different types of naming authorities.

iSCSI names are associated with iSCSI nodes, and not iSCSI network
adapter cards, to ensure that the replacement of network adapter
cards does not require reconfiguration of all SCSI and iSCSI resource
allocation information.

Some SCSI commands require that protocol-specific identifiers be
communicated within SCSI CDBs. See Section 2.2 for the definition of
the SCSI port name/identifier for iSCSI ports.

An initiator may discover the iSCSI Target Names to which it has
access, along with their addresses, using the SendTargets Text
Request, or other techniques discussed in [RFC3721].

iSCSI equipment that needs discovery functions beyond SendTargets
SHOULD implement iSNS (see [RFC4171]) for extended discovery
management capabilities and interoperability. Although [RFC3721]
implies an SLP ([RFC2608]) implementation requirement, SLP has not
been widely implemented or deployed for use with iSCSI in practice.
iSCSI implementations therefore SHOULD NOT rely on SLP-based
discovery interoperability.

4.2.7.1. iSCSI Name Properties

Each iSCSI node, whether it is an initiator, a target, or both, MUST
have an iSCSI name. Whenever an iSCSI node contains an iSCSI
initiator node and an iSCSI target node, the iSCSI Initiator Name
MUST be the same as the iSCSI Target Name for the contained Nodes
such that there is only one iSCSI Node Name for the iSCSI node
overall. Note the related requirements in Section 9.2.1 on how to
map CHAP names to iSCSI names in such a scenario.

Initiators and targets MUST support the receipt of iSCSI names of up
to the maximum length of 223 bytes.

The initiator MUST present both its iSCSI Initiator Name and the
iSCSI Target Name to which it wishes to connect in the first Login
Request of a new session or connection. The only exception is if a
Discovery session (see Section 4.3) is to be established. In this
case, the iSCSI Initiator Name is still required, but the iSCSI
Target Name MAY be omitted.

iSCSI names have the following properties:

iSCSI names are globally unique. No two initiators or targets
can have the same name.

iSCSI names are permanent. An iSCSI initiator node or target
node has the same name for its lifetime.

iSCSI names do not imply a location or address. An iSCSI
initiator or target can move or have multiple addresses. A
change of address does not imply a change of name.

iSCSI names do not rely on a central name broker; the naming
authority is distributed.

iSCSI names support integration with existing unique naming
schemes.

iSCSI names rely on existing naming authorities. iSCSI does not
create any new naming authority.

The encoding of an iSCSI name has the following properties:

iSCSI names have the same encoding method, regardless of the
underlying protocols.

iSCSI names are relatively simple to compare. The algorithm for
comparing two iSCSI names for equivalence does not rely on an
external server.

iSCSI names are composed only of printable ASCII and Unicode
characters. iSCSI names allow the use of international
character sets, but uppercase characters are prohibited. The
iSCSI stringprep profile [RFC3722] maps uppercase characters to
lowercase and SHOULD be used to prepare iSCSI names from input
that may include uppercase characters. No whitespace characters
are used in iSCSI names; see [RFC3722] for details.

iSCSI names may be transported using both binary and ASCII-based
protocols.

An iSCSI name really names a logical software entity and is not tied
to a port or other hardware that can be changed. For instance, an
Initiator Name should name the iSCSI initiator node, not a particular
NIC or HBA. When multiple NICs are used, they should generally all
present the same iSCSI Initiator Name to the targets, because they
are simply paths to the same SCSI layer. In most operating systems,
the named entity is the operating system image.

Similarly, a target name should not be tied to hardware interfaces
that can be changed. A target name should identify the logical
target and must be the same for the target, regardless of the
physical portion being addressed. This assists iSCSI initiators in
determining that the two targets it has discovered are really two
paths to the same target.

The iSCSI name is designed to fulfill the functional requirements for
Uniform Resource Names (URNs) [RFC1737]. For example, it is required
that the name have a global scope, be independent of address or
location, and be persistent and globally unique. Names must be
extensible and scalable with the use of naming authorities. The name
encoding should be both human and machine readable. See [RFC1737]
for further requirements.

4.2.7.2. iSCSI Name Encoding

An iSCSI name MUST be a UTF-8 (see [RFC3629]) encoding of a string of
Unicode characters with the following properties:

It is in Normalization Form C (see "Unicode Normalization Forms"
[UNICODE]).

It only contains characters allowed by the output of the iSCSI
stringprep template (described in [RFC3722]).

The following characters are used for formatting iSCSI names:

dash ('-'=U+002d)
dot ('.'=U+002e)
colon (':'=U+003a)

The UTF-8 encoding of the name is not larger than 223 bytes.

The stringprep process is described in [RFC3454]; iSCSI's use of the
stringprep process is described in [RFC3722]. The stringprep process
is a method designed by the Internationalized Domain Name (IDN)
working group to translate human-typed strings into a format that can
be compared as opaque strings. iSCSI names are expected to be used
by administrators for purposes such as system configuration; for this
reason, characters that may lead to human confusion among different
iSCSI names (e.g., punctuation, spacing, diacritical marks) should be
avoided, even when such characters are allowed as stringprep
processing output by [RFC3722]. The stringprep process also converts
strings into equivalent strings of lowercase characters.

The stringprep process does not need to be implemented if the names
are generated using only characters allowed as output by the
stringprep processing specified in [RFC3722]. Those allowed
characters include all ASCII lowercase and numeric characters, as
well as lowercase Unicode characters as specified in [RFC3722]. Once
iSCSI names encoded in UTF-8 are "normalized" as described in this
section, they may be safely compared byte for byte.

4.2.7.3. iSCSI Name Structure

An iSCSI name consists of two parts -- a type designator followed by
a unique name string.

iSCSI uses three existing naming authorities in constructing globally
unique iSCSI names. The type designator in an iSCSI name indicates
the naming authority on which the name is based. The three iSCSI
name formats are the following:

a) iSCSI-Qualified Name: based on domain names to identify a

naming authority

b) NAA format Name: based on a naming format defined by [FC-FS3]

for constructing globally unique identifiers, referred to as
the Network Address Authority (NAA)

Address Authority identifier, in ASCII-encoded hexadecimal

c) eui. - Remainder of the string is an IEEE EUI-64 identifier, in

ASCII-encoded hexadecimal

These three naming authority designators were considered sufficient
at the time of writing this document. The creation of additional
naming type designators for iSCSI may be considered by the IETF and
detailed in separate RFCs.

The following table summarizes the current SCSI transport protocols
and their naming formats.

4.2.7.4. Type "iqn." (iSCSI Qualified Name)

This iSCSI name type can be used by any organization that owns a
domain name. This naming format is useful when an end user or
service provider wishes to assign iSCSI names for targets and/or
initiators.

To generate names of this type, the person or organization generating
the name must own a registered domain name. This domain name does
not have to resolve to an address; it just needs to be reserved to
prevent others from generating iSCSI names using the same
domain name.

Since a domain name can expire, be acquired by another entity, or may
be used to generate iSCSI names by both owners, the domain name must
be additionally qualified by a date during which the naming authority
owned the domain name. A date code is provided as part of the "iqn."
format for this reason.

The iSCSI qualified name string consists of:

The string "iqn.", used to distinguish these names from "eui."
formatted names.

A date code, in yyyy-mm format. This date MUST be a date during
which the naming authority owned the domain name used in this
format and SHOULD be the first month in which the domain name
was owned by this naming authority at 00:01 GMT of the first day
of the month. This date code uses the Gregorian calendar. All
four digits in the year must be present. Both digits of the
month must be present, with January == "01" and December ==
"12". The dash must be included.

A dot "."

The reverse domain name of the naming authority (person or
organization) creating this iSCSI name.

- An optional, colon (:)-prefixed string within the character set
and length boundaries that the owner of the domain name deems
appropriate. This may contain product types, serial numbers,
host identifiers, or software keys (e.g., it may include colons
to separate organization boundaries). With the exception of the
colon prefix, the owner of the domain name can assign everything
after the reverse domain name as desired. It is the
responsibility of the entity that is the naming authority to
ensure that the iSCSI names it assigns are worldwide unique.
For example, "Example Storage Arrays, Inc." might own the domain
name "example.com".

The following are examples of iSCSI qualified names that might be
generated by "EXAMPLE Storage Arrays, Inc."

4.2.7.5. Type "eui." (IEEE EUI-64 Format)

The IEEE Registration Authority provides a service for assigning
globally unique identifiers [EUI]. The EUI-64 format is used to
build a global identifier in other network protocols. For example,
Fibre Channel defines a method of encoding it into a WorldWideName.
For more information on registering for EUI identifiers, see [OUI].

The format is "eui." followed by an EUI-64 identifier (16 ASCII-
encoded hexadecimal digits).

4.2.7.6. Type "naa." (Network Address Authority)

The INCITS T11 Framing and Signaling Specification [FC-FS3] defines a
format called the Network Address Authority (NAA) format for
constructing worldwide unique identifiers that use various identifier
registration authorities. This identifier format is used by the
Fibre Channel and SAS SCSI transport protocols. As FC and SAS
constitute a large fraction of networked SCSI ports, the NAA format
is a widely used format for SCSI transports. The objective behind
iSCSI supporting a direct representation of an NAA format Name is to
facilitate construction of a target device name that translates
easily across multiple namespaces for a SCSI storage device
containing ports served by different transports. More specifically,
this format allows implementations wherein one NAA identifier can be
assigned as the basis for the SCSI device name for a SCSI target with
both SAS ports and iSCSI ports.

The iSCSI NAA naming format is "naa.", followed by an NAA identifier
represented in ASCII-encoded hexadecimal digits.

The iSCSI NAA naming format might be used in an implementation when
the infrastructure for generating NAA worldwide unique names is
already in place because the device contains both SAS and iSCSI SCSI
ports.

The NAA identifier formatted in an ASCII-hexadecimal representation
has a maximum size of 32 characters (128-bit NAA format). As a
result, there is no issue with this naming format exceeding the
maximum size for iSCSI Node Names.

4.2.8. Persistent State

iSCSI does not require any persistent state maintenance across
sessions. However, in some cases, SCSI requires persistent
identification of the SCSI initiator port name (see Sections 4.4.2
and 4.4.3.)

iSCSI sessions do not persist through power cycles and boot
operations.

All iSCSI session and connection parameters are reinitialized on
session and connection creation.

Commands persist beyond connection termination if the session
persists and command recovery within the session is supported.
However, when a connection is dropped, command execution, as
perceived by iSCSI (i.e., involving iSCSI protocol exchanges for the
affected task), is suspended until a new allegiance is established by
the "TASK REASSIGN" task management function. See Section 11.5.

4.2.9. Message Synchronization and Steering

iSCSI presents a mapping of the SCSI protocol onto TCP. This
encapsulation is accomplished by sending iSCSI PDUs of varying
lengths. Unfortunately, TCP does not have a built-in mechanism for
signaling message boundaries at the TCP layer. iSCSI overcomes this
obstacle by placing the message length in the iSCSI message header.
This serves to delineate the end of the current message as well as
the beginning of the next message.

In situations where IP packets are delivered in order from the
network, iSCSI message framing is not an issue and messages are
processed one after the other. In the presence of IP packet
reordering (i.e., frames being dropped), legacy TCP implementations
store the "out of order" TCP segments in temporary buffers until the
missing TCP segments arrive, at which time the data must be copied to
the application buffers. In iSCSI, it is desirable to steer the SCSI
data within these out-of-order TCP segments into the preallocated
SCSI buffers rather than store them in temporary buffers. This
decreases the need for dedicated reassembly buffers as well as the
latency and bandwidth related to extra copies.

Relying solely on the "message length" information from the iSCSI
message header may make it impossible to find iSCSI message
boundaries in subsequent TCP segments due to the loss of a TCP
segment that contains the iSCSI message length. The missing TCP
segment(s) must be received before any of the following segments can
be steered to the correct SCSI buffers (due to the inability to
determine the iSCSI message boundaries). Since these segments cannot
be steered to the correct location, they must be saved in temporary
buffers that must then be copied to the SCSI buffers.

Different schemes can be used to recover synchronization. The
details of any such schemes are beyond this protocol specification,
but it suffices to note that [RFC4297] provides an overview of the
direct data placement problem on IP networks, and [RFC5046] specifies
a protocol extension for iSCSI that facilitates this direct data
placement objective. The rest of this document refers to any such
direct data placement protocol usage as an example of a "Sync and
Steering layer".

Under normal circumstances (no PDU loss or data reception out of
order), iSCSI data steering can be accomplished by using the
identifying tag and the data offset fields in the iSCSI header in
addition to the TCP sequence number from the TCP header. The
identifying tag helps associate the PDU with a SCSI buffer address,
while the data offset and TCP sequence number are used to determine
the offset within the buffer.

4.2.9.1. Sync/Steering and iSCSI PDU Length

When a large iSCSI message is sent, the TCP segment(s) that contains
the iSCSI header may be lost. The remaining TCP segment(s) up to the
next iSCSI message must be buffered (in temporary buffers) because
the iSCSI header that indicates to which SCSI buffers the data are to
be steered was lost. To minimize the amount of buffering, it is
recommended that the iSCSI PDU length be restricted to a small value
(perhaps a few TCP segments in length). During login, each end of
the iSCSI session specifies the maximum iSCSI PDU length it will
accept.

4.3. iSCSI Session Types

iSCSI defines two types of sessions:

a) Normal operational session - an unrestricted session.

b) Discovery session - a session only opened for target discovery.

The target MUST ONLY accept Text Requests with the SendTargets
key and a Logout Request with reason "close the session". All
other requests MUST be rejected.

The session type is defined during login with the SessionType=value
parameter in the login command.

4.4. SCSI-to-iSCSI Concepts Mapping Model

The following diagram shows an example of how multiple iSCSI nodes
(targets in this case) can coexist within the same Network Entity and
can share Network Portals (IP addresses and TCP ports). Other more
complex configurations are also possible. For detailed descriptions
of the components of these diagrams, see Section 4.4.1.

4.4.1. iSCSI Architecture Model

This section describes the part of the iSCSI Architecture Model that
has the most bearing on the relationship between iSCSI and the SCSI
Architecture Model.

Network Entity - represents a device or gateway that is
accessible from the IP network. A Network Entity must have one
or more Network Portals (see the "Network Portal" item below),
each of which can be used by some iSCSI nodes (see the next
item) contained in that Network Entity to gain access to the IP
network.

iSCSI Node - represents a single iSCSI initiator or iSCSI
target, or an instance of each. There are one or more iSCSI
nodes within a Network Entity. The iSCSI node is accessible via
one or more Network Portals (see below). An iSCSI node is
identified by its iSCSI name (see Sections 4.2.7 and 13). The
separation of the iSCSI name from the addresses used by and for
the iSCSI node allows multiple iSCSI nodes to use the same
addresses and allows the same iSCSI node to use multiple
addresses.

An alias string may also be associated with an iSCSI node. The
alias allows an organization to associate a user-friendly string
with the iSCSI name. However, the alias string is not a
substitute for the iSCSI name.

Network Portal - a component of a Network Entity that has a
TCP/IP network address and that may be used by an iSCSI node
within that Network Entity for the connection(s) within one of
its iSCSI sessions. In an initiator, it is identified by its IP
address. In a target, it is identified by its IP address and
its listening TCP port.

Portal Groups - iSCSI supports multiple connections within the
same session; some implementations will have the ability to
combine connections in a session across multiple Network
Portals. A portal group defines a set of Network Portals within
an iSCSI node that collectively supports the capability of
coordinating a session with connections that span these portals.
Not all Network Portals within a portal group need to
participate in every session connected through that portal
group. One or more portal groups may provide access to an iSCSI
node. Each Network Portal, as utilized by a given iSCSI node,
belongs to exactly one portal group within that node. Portal
groups are identified within an iSCSI node by a Portal Group
Tag, a simple unsigned integer between 0 and 65535 (see

Section 13.9). All Network Portals with the same Portal Group
Tag in the context of a given iSCSI node are in the same portal
group.

Both iSCSI initiators and iSCSI targets have portal groups,
though only the iSCSI target portal groups are used directly in
the iSCSI protocol (e.g., in SendTargets). For references to
the initiator portal Groups, see Section 10.1.2.

Portals within a portal group should support similar session
parameters, because they may participate in a common session.

The following diagram shows an example of one such configuration on a
target and how a session that shares Network Portals within a portal
group may be established.

4.4.2. SCSI Architecture Model

This section describes the relationship between the SCSI Architecture
Model [SAM2] and constructs of the SCSI device, SCSI port and I_T
nexus, and the iSCSI constructs described in Section 4.4.1.

This relationship implies implementation requirements in order to
conform to the SAM-2 model and other SCSI operational functions.

These requirements are detailed in Section 4.4.3.

The following list outlines mappings of SCSI architectural elements
to iSCSI.

a) SCSI Device - This is the SAM-2 term for an entity that

contains one or more SCSI ports that are connected to a service
delivery subsystem and supports a SCSI application protocol.
For example, a SCSI initiator device contains one or more SCSI
initiator ports and zero or more application clients. A SCSI
target device contains one or more SCSI target ports and one or
more LUs. For iSCSI, the SCSI device is the component within
an iSCSI node that provides the SCSI functionality. As such,
there can be at most one SCSI device within an iSCSI node.
Access to the SCSI device can only be achieved in an iSCSI
Normal operational session (see Section 4.3). The SCSI device
name is defined to be the iSCSI name of the node and MUST be
used in the iSCSI protocol.

b) SCSI Port - This is the SAM-2 term for an entity in a SCSI

device that provides the SCSI functionality to interface with a
service delivery subsystem or transport. For iSCSI, the
definitions of the SCSI initiator port and the SCSI target port
are different.

SCSI initiator port: This maps to one endpoint of an iSCSI
Normal operational session (see Section 4.3). An iSCSI Normal
operational session is negotiated through the login process
between an iSCSI initiator node and an iSCSI target node. At
successful completion of this process, a SCSI initiator port is
created within the SCSI initiator device. The SCSI initiator
port Name and SCSI initiator port Identifier are both defined
to be the iSCSI Initiator Name together with (a) a label that
identifies it as an initiator port name/identifier and (b) the
ISID portion of the session identifier.

SCSI target port: This maps to an iSCSI target portal group.
The SCSI Target Port Name and the SCSI Target Port Identifier
are both defined to be the iSCSI Target Name together with (a)
a label that identifies it as a target port name/identifier and
(b) the Target Portal Group Tag.

The SCSI port name MUST be used in iSCSI. When used in SCSI
parameter data, the SCSI port name MUST be encoded as:

the iSCSI name in UTF-8 format, followed by

a comma separator (1 byte), followed by

the ASCII character 'i' (for SCSI initiator port) or the
ASCII character 't' (for SCSI target port) (1 byte),
followed by

a comma separator (1 byte), followed by

a text encoding as a hex-constant (see Section 6.1) of the
ISID (for SCSI initiator port) or the Target Portal Group
Tag (for SCSI target port), including the initial 0X or 0x
and the terminating null (15 bytes for iSCSI initiator port,
7 bytes for iSCSI target port).

The ASCII character 'i' or 't' is the label that identifies
this port as either a SCSI initiator port or a SCSI target
port.

c) I_T nexus - This indicates a relationship between a SCSI

initiator port and a SCSI target port, according to [SAM2].
For iSCSI, this relationship is a session, defined as a
relationship between an iSCSI initiator's end of the session
(SCSI initiator port) and the iSCSI target's portal group. The
I_T nexus can be identified by the conjunction of the SCSI port
names or by the iSCSI session identifier (SSID). iSCSI defines
the I_T nexus identifier to be the tuple (iSCSI Initiator Name
+ ",i,0x" + ISID in text format, iSCSI Target Name + ",t,0x" +
Target Portal Group Tag in text format). An uppercase hex
prefix "0X" may alternatively be used in place of "0x".

NOTE: The I_T nexus identifier is not equal to the SSID.

4.4.3. Consequences of the Model

This section describes implementation and behavioral requirements
that result from the mapping of SCSI constructs to the iSCSI
constructs defined above. Between a given SCSI initiator port and a
given SCSI target port, only one I_T nexus (session) can exist. No
more than one nexus relationship (parallel nexus) is allowed by
[SAM2]. Therefore, at any given time, only one session with the same
SSID can exist between a given iSCSI initiator node and an iSCSI
target node.

These assumptions lead to the following conclusions and requirements:

ISID RULE: Between a given iSCSI initiator and iSCSI target portal
group (SCSI target port), there can only be one session with a given
value for the ISID that identifies the SCSI initiator port. See
Section 11.12.5.

The structure of the ISID that contains a naming authority component
(see Section 11.12.5 and [RFC3721]) provides a mechanism to
facilitate compliance with the ISID RULE. See Section 10.1.1.

The iSCSI initiator node should manage the assignment of ISIDs prior
to session initiation. The "ISID RULE" does not preclude the use of
the same ISID from the same iSCSI initiator with different target
portal groups on the same iSCSI target or on other iSCSI targets (see
Section 10.1.1). Allowing this would be analogous to a single SCSI
initiator port having relationships (nexus) with multiple SCSI target
ports on the same SCSI target device or SCSI target ports on other
SCSI target devices. It is also possible to have multiple sessions
with different ISIDs to the same target portal group. Each such
session would be considered to be with a different initiator even
when the sessions originate from the same initiator device. The same
ISID may be used by a different iSCSI initiator because it is the
iSCSI name together with the ISID that identifies the SCSI initiator
port.

NOTE: A consequence of the ISID RULE and the specification for the
I_T nexus identifier is that two nexuses with the same identifier
should never exist at the same time.

TSIH RULE: The iSCSI target selects a non-zero value for the TSIH at
session creation (when an initiator presents a 0 value at login).
After being selected, the same TSIH value MUST be used whenever the
initiator or target refers to the session and a TSIH is required.

4.4.3.1. I_T Nexus State

Certain nexus relationships contain an explicit state (e.g.,
initiator-specific mode pages) that may need to be preserved by the
device server [SAM2] in a LU through changes or failures in the iSCSI
layer (e.g., session failures). In order for that state to be
restored, the iSCSI initiator should reestablish its session
(re-login) to the same target portal group using the previous ISID.
That is, it should reinstate the session via iSCSI session
reinstatement (Section 6.3.5) or continue via session continuation
(Section 6.3.6). This is because the SCSI initiator port identifier
and the SCSI target port identifier (or relative target port) form
the datum that the SCSI LU device server uses to identify the I_T
nexus.

4.4.3.2. Reservations

There are two reservation management methods defined in the SCSI
standards: reserve/release reservations, based on the RESERVE and
RELEASE commands [SPC2]; and persistent reservations, based on the
PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT commands [SPC3].
Reserve/release reservations are obsolete [SPC3] and should not be
used. Persistent reservations are suggested as an alternative; see
Annex B of [SPC4].

State for persistent reservations is required to persist through
changes and failures at the iSCSI layer that result in I_T nexus
failures; see [SPC3] for details and specific requirements.

In contrast, [SPC2] does not specify detailed persistence
requirements for reserve/release reservation state after an I_T nexus
failure. Nonetheless, when reserve/release reservations are
supported by an iSCSI target, the preferred implementation approach
is to preserve reserve/release reservation state for iSCSI session
reinstatement (see Section 6.3.5) or session continuation (see
Section 6.3.6).

Two additional caveats apply to reserve/release reservations:

Retention of a failed session's reserve/release reservation
state by an iSCSI target, even after that failed iSCSI session
is not reinstated or continued, may require an initiator to
issue a reset (e.g., LOGICAL UNIT RESET; see Section 11.5) in
order to remove that reservation state.

Reserve/release reservations may not behave as expected when
persistent reservations are also used on the same LU; see the
discussion of "Exceptions to SPC-2 RESERVE and RELEASE behavior"
in [SPC4].

4.5. iSCSI UML Model

This section presents the application of the UML modeling concepts
discussed in Section 3 to the iSCSI and SCSI Architecture Model
discussed in Section 4.4.

target node instance, one iSCSI initiator node instance, or both.
However, in all scenarios, note that an iSCSI node MUST only have
a single iSCSI name. Note the related requirement in
Section 4.2.7.1.

4.6. Request/Response Summary

This section lists and briefly describes all the iSCSI PDU types
(requests and responses).

All iSCSI PDUs are built as a set of one or more header segments
(basic and auxiliary) and zero or one data segments. The header
group and the data segment may each be followed by a CRC (digest).

The basic header segment has a fixed length of 48 bytes.

4.6.1. Request/Response Types Carrying SCSI Payload

4.6.1.1. SCSI Command

This request carries the SCSI CDB and all the other SCSI Execute
Command [SAM2] procedure call IN arguments, such as task attributes,
Expected Data Transfer Length for one or both transfer directions
(the latter for bidirectional commands), and a task tag (as part of
the I_T_L_x nexus). The I_T_L nexus is derived by the initiator and
target from the LUN field in the request, and the I_T nexus is
implicit in the session identification.

In addition, the SCSI Command PDU carries information required for
the proper operation of the iSCSI protocol -- the command sequence
number (CmdSN) and the expected status sequence number (ExpStatSN) on
the connection it is issued.

All or part of the SCSI output (write) data associated with the SCSI
command may be sent as part of the SCSI Command PDU as a data
segment.

4.6.1.2. SCSI Response

The SCSI Response contains the residual counts from the operation, if
any; an indication of whether the counts represent an overflow or an
underflow; and the SCSI status if the status is valid or a response
code (a non-zero return value for the Execute Command procedure call)
if the status is not valid.

For a valid status that indicates that the command has been processed
but resulted in an exception (e.g., a SCSI CHECK CONDITION), the PDU
data segment contains the associated sense data. The use of
Autosense ([SAM2]) is REQUIRED by iSCSI.

Some data segment content may also be associated (in the data
segment) with a non-zero response code.

In addition, the SCSI Response PDU carries information required for
the proper operation of the iSCSI protocol:

ExpDataSN - the number of Data-In PDUs that a target has sent
(to enable the initiator to check that all have arrived)

StatSN - the status sequence number on this connection

ExpCmdSN - the next expected command sequence number at the
target

MaxCmdSN - the maximum CmdSN acceptable at the target from this
initiator

4.6.1.3. Task Management Function Request

The Task Management Function Request provides an initiator with a way
to explicitly control the execution of one or more SCSI tasks or
iSCSI functions. The PDU carries a function identifier (i.e., which
task management function to perform) and enough information to
unequivocally identify the task or task set on which to perform the
action, even if the task(s) to act upon has not yet arrived or has
been discarded due to an error.

The referenced tag identifies an individual task if the function
refers to an individual task.

The I_T_L nexus identifies task sets. In iSCSI, the I_T_L nexus is
identified by the LUN and the session identification (the session
identifies an I_T nexus).

For task sets, the CmdSN of the Task Management Function Request
helps identify the tasks upon which to act, namely all tasks
associated with a LUN and having a CmdSN preceding the Task
Management Function Request CmdSN.

For a task management function, the coordination between responses to
the tasks affected and the Task Management Function Response is done
by the target.

4.6.1.4. Task Management Function Response

The Task Management Function Response carries an indication of
function completion for a Task Management Function Request, including
how it completed (response and qualifier) and additional information
for failure responses.

After the Task Management Function Response indicates task management
function completion, the initiator will not receive any additional
responses from the affected tasks.

4.6.1.5. SCSI Data-Out and SCSI Data-In

SCSI Data-Out and SCSI Data-In are the main vehicles by which SCSI
data payload is carried between the initiator and target. Data
payload is associated with a specific SCSI command through the
Initiator Task Tag. For target convenience, outgoing solicited data
also carries a Target Transfer Tag (copied from R2T) and the LUN.
Each PDU contains the payload length and the data offset relative to
the buffer address contained in the SCSI Execute Command procedure
call.

In each direction, the data transfer is split into "sequences". An
end-of-sequence is indicated by the F bit.

An outgoing sequence is either unsolicited (only the first sequence
can be unsolicited) or consists of all the Data-Out PDUs sent in
response to an R2T.

Input sequences enable the switching of direction for bidirectional
commands as required.

For input, the target may request positive acknowledgment of input
data. This is limited to sessions that support error recovery and is
implemented through the A bit in the SCSI Data-In PDU header.

Data-In and Data-Out PDUs also carry the DataSN to enable the
initiator and target to detect missing PDUs (discarded due to an
error).

In addition, the StatSN is carried by the Data-In PDUs.

To enable a SCSI command to be processed while involving a minimum
number of messages, the last SCSI Data-In PDU passed for a command
may also contain the status if the status indicates termination with
no exceptions (no sense or response involved).

4.6.1.6. Ready To Transfer (R2T)

R2T is the mechanism by which the SCSI target "requests" the
initiator for output data. R2T specifies to the initiator the offset
of the requested data relative to the buffer address from the Execute
Command procedure call and the length of the solicited data.

To help the SCSI target associate the resulting Data-Out with an R2T,
the R2T carries a Target Transfer Tag that will be copied by the
initiator in the solicited SCSI Data-Out PDUs. There are no
protocol-specific requirements with regard to the value of these
tags, but it is assumed that together with the LUN, they will enable
the target to associate data with an R2T.

R2T also carries information required for proper operation of the
iSCSI protocol, such as:

Text Requests/Responses may form extended sequences using the same
Initiator Task Tag. The initiator uses the F (Final) flag bit in the
Text Request header to indicate its readiness to terminate a
sequence. The target uses the F bit in the Text Response header to
indicate its consent to sequence termination.

Text Requests and Responses also use the Target Transfer Tag to
indicate continuation of an operation or a new beginning. A target
that wishes to continue an operation will set the Target Transfer Tag
in a Text Response to a value different from the default 0xffffffff.
An initiator willing to continue will copy this value into the Target
Transfer Tag of the next Text Request. If the initiator wants to
restart the current target negotiation (start fresh), it will set the
Target Transfer Tag to 0xffffffff.

Although a complete exchange is always started by the initiator,
specific parameter negotiations may be initiated by the initiator or
target.

4.6.3.2. Login Requests and Login Responses

Login Requests and Responses are used exclusively during the Login
Phase of each connection to set up the session and connection
parameters. (The Login Phase consists of a sequence of Login
Requests and Responses carrying the same Initiator Task Tag.)

A connection is identified by an arbitrarily selected connection ID
(CID) that is unique within a session.

Similar to the Text Requests and Responses, Login Requests/Responses
carry key=value text information with a simple syntax in the data
segment.

The Login Phase proceeds through several stages (security
negotiation, operational parameter negotiation) that are selected
with two binary coded fields in the header -- the Current Stage (CSG)
and the Next Stage (NSG) -- with the appearance of the latter being
signaled by the "Transit" flag (T).

The first Login Phase of a session plays a special role, called the
leading login, which determines some header fields (e.g., the version
number, the maximum number of connections, and the session
identification).

The CmdSN initial value is also set by the leading login.

The StatSN for each connection is initiated by the connection login.

A Login Request may indicate an implied logout (cleanup) of the
connection to be logged in (a connection restart) by using the same
connection ID (CID) as an existing connection as well as the same
session-identifying elements of the session to which the old
connection was associated.

4.6.3.3. Logout Requests and Logout Responses

Logout Requests and Responses are used for the orderly closing of
connections for recovery or maintenance. The Logout Request may be
issued following a target prompt (through an Asynchronous Message) or
at an initiator's initiative. When issued on the connection to be
logged out, no other request may follow it.

The Logout Response indicates that the connection or session cleanup
is completed and no other responses will arrive on the connection (if
received on the logging-out connection). In addition, the Logout
Response indicates how long the target will continue to hold
resources for recovery (e.g., command execution that continues on a
new connection) in the Time2Retain field and how long the initiator
must wait before proceeding with recovery in the Time2Wait field.

4.6.3.4. SNACK Request

With the SNACK Request, the initiator requests retransmission of
numbered responses or data from the target. A single SNACK Request
covers a contiguous set of missing items, called a run, of a given
type of items. The type is indicated in a type field in the PDU
header. The run is composed of an initial item (StatSN, DataSN,
R2TSN) and the number of missed Status, Data, or R2T PDUs. For long
Data-In sequences, the target may request (at predefined minimum
intervals) a positive acknowledgment for the data sent. A SNACK
Request with a type field that indicates ACK and the number of
Data-In PDUs acknowledged conveys this positive acknowledgment.

4.6.3.5. Reject

Reject enables the target to report an iSCSI error condition (e.g.,
protocol, unsupported option) that uses a Reason field in the PDU
header and includes the complete header of the bad PDU in the Reject
PDU data segment.

4.6.3.6. NOP-Out Request and NOP-In Response

This request/response pair may be used by an initiator and target as
a "ping" mechanism to verify that a connection/session is still
active and all of its components are operational. Such a ping may be
triggered by the initiator or target. The triggering party indicates
that it wants a reply by setting a value different from the default
0xffffffff in the corresponding Initiator/Target Transfer Tag.

NOP-In/NOP-Out may also be used in "unidirectional" fashion to convey
to the initiator/target command, status, or data counter values when
there is no other "carrier" and there is a need to update the
initiator/target.

5. SCSI Mode Parameters for iSCSI

There are no iSCSI-specific mode pages.

6. Login and Full Feature Phase Negotiation

iSCSI parameters are negotiated at session or connection
establishment by using Login Requests and Responses (see
Section 4.2.4) and during the Full Feature Phase (Section 4.2.5) by
using Text Requests and Responses. In both cases, the mechanism used
is an exchange of iSCSI-text-key=value pairs. For brevity,
iSCSI-text-keys are called just "keys" in the rest of this document.

Keys are either declarative or require negotiation, and the key
description indicates whether the key is declarative or requires
negotiation.

For the declarative keys, the declaring party sets a value for the
key. The key specification indicates whether the key can be declared
by the initiator, the target, or both.

For the keys that require negotiation, one of the parties (the
proposing party) proposes a value or set of values by including the
key=value in the data part of a Login or Text Request or Response.
The other party (the accepting party) makes a selection based on the
value or list of values proposed and includes the selected value in a
key=value in the data part of the following Login or Text Response or
Request. For most of the keys, both the initiator and target can be
proposing parties.

The login process proceeds in two stages -- the security negotiation
stage and the operational parameter negotiation stage. Both stages
are optional, but at least one of them has to be present to enable
setting some mandatory parameters.

Progression from stage to stage is controlled by the T (Transit) bit
in the Login Request/Response PDU header. Through the T bit set
to 1, the initiator indicates that it would like to transition. The
target agrees to the transition (and selects the next stage) when
ready. A field in the Login PDU header indicates the current stage
(CSG), and during transition, another field indicates the next stage
(NSG) proposed (initiator) and selected (target).

The text negotiation process is used to negotiate or declare
operational parameters. The negotiation process is controlled by the
F (Final) bit in the PDU header. During text negotiations, the F bit
is used by the initiator to indicate that it is ready to finish the
negotiation and by the target to acquiesce the end of negotiation.

Since some key=value pairs may not fit entirely in a single PDU, the
C (Continue) bit is used (both in Login and Text) to indicate that
"more follows".

The text negotiation uses an additional mechanism by which a target
may deliver larger amounts of data to an inquiring initiator. The
target sets a Target Task Tag to be used as a bookmark that, when
returned by the initiator, means "go on". If reset to a "neutral
value", it means "forget about the rest".

This section details the types of keys and values used, the syntax
rules for parameter formation, and the negotiation schemes to be used
with different types of parameters.

6.1. Text Format

The initiator and target send a set of key=value pairs encoded in
UTF-8 Unicode. All the text keys and text values specified in this
document are case sensitive; they are to be presented and interpreted
as they appear in this document without change of case.

The following character symbols are used in this document for text
items (the hexadecimal values represent Unicode code points):

Key=value pairs may span PDU boundaries. An initiator or target that
sends partial key=value text within a PDU indicates that more text
follows by setting the C bit in the Text or Login Request or the Text
or Login Response to 1. Data segments in a series of PDUs that have
the C bit set to 1 and end with a PDU that has the C bit set to 0, or
that include a single PDU that has the C bit set to 0, have to be
considered as forming a single logical-text-data-segment (LTDS).

Every key=value pair, including the last or only pair in a LTDS, MUST
be followed by one null (0x00) delimiter.

A key-name is whatever precedes the first "=" in the key=value pair.
The term "key" is used frequently in this document in place of
"key-name".

A value is whatever follows the first "=" in the key=value pair up to
the end of the key=value pair, but not including the null delimiter.

The following definitions will be used in the rest of this document:

standard-label: A string of one or more characters that consists
of letters, digits, dot, minus, plus, commercial at, or
underscore. A standard-label MUST begin with a capital letter
and must not exceed 63 characters.

key-name: A standard-label.

text-value: A string of zero or more characters that consists of
letters, digits, dot, minus, plus, commercial at, underscore,
slash, left bracket, right bracket, or colon.

iSCSI-name-value: A string of one or more characters that
consists of minus, dot, colon, or any character allowed by the
output of the iSCSI stringprep template as specified in
[RFC3722] (see also Section 4.2.7.2).

- iSCSI-local-name-value: A UTF-8 string; no null characters are
allowed in the string. This encoding is to be used for
localized (internationalized) aliases.

boolean-value: The string "Yes" or "No".

hex-constant: A hexadecimal constant encoded as a string that
starts with "0x" or "0X" followed by one or more digits or the
letters a, b, c, d, e, f, A, B, C, D, E, or F. Hex-constants
are used to encode numerical values or binary strings. When
used to encode numerical values, the excessive use of leading 0
digits is discouraged. The string following 0X (or 0x)
represents a base16 number that starts with the most significant
base16 digit, followed by all other digits in decreasing order
of significance and ending with the least significant base16
digit. When used to encode binary strings, hexadecimal
constants have an implicit byte-length that includes four bits
for every hexadecimal digit of the constant, including leading
zeroes. For example, a hex-constant of n hexadecimal digits has
a byte-length of (the integer part of) (n + 1)/2.

decimal-constant: An unsigned decimal number with the digit 0 or
a string of one or more digits that starts with a non-zero
digit. Decimal-constants are used to encode numerical values or
binary strings. Decimal-constants can only be used to encode
binary strings if the string length is explicitly specified.
There is no implicit length for decimal strings.
Decimal-constants MUST NOT be used for parameter values if the
values can be equal to or greater than 2**64 (numerical) or for
binary strings that can be longer than 64 bits.

base64-constant: Base64 constant encoded as a string that starts
with "0b" or "0B" followed by 1 or more digits, letters, plus
sign, slash, or equals sign. The encoding is done according to
[RFC4648].

- numerical-value: An unsigned integer always less than 2**64
encoded as a decimal-constant or a hex-constant. Unsigned
integer arithmetic applies to numerical-values.
- large-numerical-value: An unsigned integer that can be larger
than or equal to 2**64 encoded as a hex-constant or
base64-constant. Unsigned integer arithmetic applies to large-
numerical-values.

numerical-range: Two numerical-values separated by a tilde,
where the value to the right of the tilde must not be lower than
the value to the left.

regular-binary-value: A binary string not longer than 64 bits
encoded as a decimal-constant, hex-constant, or base64-constant.
The length of the string is either specified by the key
definition or is the implicit byte-length of the encoded string.

large-binary-value: A binary string longer than 64 bits encoded
as a hex-constant or base64-constant. The length of the string
is either specified by the key definition or is the implicit
byte-length of the encoded string.

binary-value: A regular-binary-value or a large-binary-value.
Operations on binary values are key-specific.

If not otherwise specified, the maximum length of a simple-value (not
its encoded representation) is 255 bytes, not including the delimiter
(comma or zero byte).

Any iSCSI target or initiator MUST support receiving at least
8192 bytes of key=value data in a negotiation sequence. When
proposing or accepting authentication methods that explicitly require
support for very long authentication items, the initiator and target
MUST support receiving at least 64 kilobytes of key=value data.

6.2. Text Mode Negotiation

During login, and thereafter, some session or connection parameters
are either declared or negotiated through an exchange of textual
information.

The initiator starts the negotiation and/or declaration through a
Text or Login Request and indicates when it is ready for completion
(by setting the F bit to 1 and keeping it at 1 in a Text Request, or
the T bit in the Login Request). As negotiation text may span PDU
boundaries, a Text or Login Request or a Text or Login Response PDU
that has the C bit set to 1 MUST NOT have the F bit or T bit set
to 1.

A target receiving a Text or Login Request with the C bit set to 1
MUST answer with a Text or Login Response with no data segment
(DataSegmentLength 0). An initiator receiving a Text or Login
Response with the C bit set to 1 MUST answer with a Text or Login
Request with no data segment (DataSegmentLength 0).

A target or initiator SHOULD NOT use a Text or Login Response or a
Text or Login Request with no data segment (DataSegmentLength 0)
unless explicitly required by a general or a key-specific negotiation
rule.

There MUST NOT be more than one outstanding Text Request, or Text
Response PDU on an iSCSI connection. An outstanding PDU in this
context is one that has not been acknowledged by the remote iSCSI
side.

Thus, a declaration is a one-way textual exchange (unless the key is
not understood by the receiver), while a negotiation is a two-way
exchange.

The proposer or declarer can be either the initiator or the target,
and the acceptor can be either the target or initiator, respectively.
Targets are not limited to respond to key=value pairs as proposed by
the initiator. The target may propose key=value pairs of its own.

All negotiations are explicit (i.e., the result MUST only be based on
newly exchanged or declared values). There are no implicit
proposals. If a proposal is not made, then a reply cannot be
expected. Conservative design also requires that default values
should not be relied upon when the use of some other value has
serious consequences.

The value proposed or declared can be a numerical-value, a numerical-
range defined by the lower and upper value with both integers
separated by a tilde, a binary value, a text-value, an iSCSI-name-
value, an iSCSI-local-name-value, a boolean-value (Yes or No), or a
list of comma-separated text-values. A range, a large-numerical-
value, an iSCSI-name-value, and an iSCSI-local-name-value MAY ONLY be
used if explicitly allowed. An accepted value can be a numerical-
value, a large-numerical-value, a text-value, or a boolean-value.

If a specific key is not relevant for the current negotiation, the
acceptor may answer with the constant "Irrelevant" for all types of
negotiations. However, the negotiation is not considered to have
failed if the answer is "Irrelevant". The "Irrelevant" answer is
meant for those cases in which several keys are presented by a
proposing party but the selection made by the acceptor for one of the
keys makes other keys irrelevant. The following example illustrates
the use of "Irrelevant":

Any key not understood by the acceptor may be ignored by the acceptor
without affecting the basic function. However, the answer for a key
that is not understood MUST be key=NotUnderstood. Note that
NotUnderstood is a valid answer for both declarative and negotiated
keys. The general iSCSI philosophy is that comprehension precedes
processing for any iSCSI key. A proposer of an iSCSI key, negotiated
or declarative, in a text key exchange MUST thus be able to properly
handle a NotUnderstood response.

The proper way to handle a NotUnderstood response depends on where
the key is specified and whether the key is declarative or
negotiated. An iSCSI implementation MUST comprehend all text keys
defined in this document. Returning a NotUnderstood response on any
of these text keys therefore MUST be considered a protocol error and
handled accordingly. For all other "later" keys, i.e., text keys
defined in later specifications, a NotUnderstood answer concludes the
negotiation for a negotiated key, whereas for a declarative key a
NotUnderstood answer simply informs the declarer of a lack of
comprehension by the receiver.

In either case, a NotUnderstood answer always requires that the
protocol behavior associated with that key not be used within the
scope of the key (connection/session) by either side.

The constants "None", "Reject", "Irrelevant", and "NotUnderstood" are
reserved and MUST ONLY be used as described here. Violation of this
rule is a protocol error (in particular, the use of "Reject",
"Irrelevant", and "NotUnderstood" as proposed values).

"Reject" or "Irrelevant" are legitimate negotiation options where
allowed, but their excessive use is discouraged. A negotiation is
considered complete when the acceptor has sent the key value pair
even if the value is "Reject", "Irrelevant", or "NotUnderstood".
Sending the key again would be a renegotiation and is forbidden for
many keys.

If the acceptor sends "Reject" as an answer, the negotiated key is
left at its current value (or default if no value was set). If the
current value is not acceptable to the proposer on the connection or
to the session in which it is sent, the proposer MAY choose to
terminate the connection or session.

All keys in this document MUST be supported by iSCSI initiators and
targets when used as specified here. If used as specified, these
keys MUST NOT be answered with NotUnderstood.

Implementers may introduce new private keys by prefixing them with X-
followed by their (reverse) domain name, or with new public keys
registered with IANA. For example, the entity owning the domain
example.com can issue:

X-com.example.bar.foo.do_something=3

Each new public key in the course of standardization MUST define the
acceptable responses to the key, including NotUnderstood as
appropriate. Unlike [RFC3720], note that this document prohibits the
X# prefix for new public keys. Based on iSCSI implementation
experience, we know that there is no longer a need for a standard
name prefix for keys that allow a NotUnderstood response. Note that
NotUnderstood will generally have to be allowed for new public keys
for backwards compatibility, as well as for private X- keys. Thus,
the name prefix "X#" in new public key-names does not carry any
significance. To avoid confusion, new public key-names MUST NOT
begin with an "X#" prefix.

Implementers MAY also introduce new values, but ONLY for new keys or
authentication methods (see Section 12) or digests (see
Section 13.1).

Whenever parameter actions or acceptance are dependent on other
parameters, the dependency rules and parameter sequence must be
specified with the parameters.

In the Login Phase (see Section 6.3), every stage is a separate
negotiation. In the Full Feature Phase, a Text Request/Response
sequence is a negotiation. Negotiations MUST be handled as atomic
operations. For example, all negotiated values go into effect after
the negotiation concludes in agreement or are ignored if the
negotiation fails.

Some parameters may be subject to integrity rules (e.g., parameter-x
must not exceed parameter-y, or parameter-u not 1 implies that
parameter-v be Yes). Whenever required, integrity rules are
specified with the keys. Checking for compliance with the integrity
rule must only be performed after all the parameters are available
(the existent and the newly negotiated). An iSCSI target MUST
perform integrity checking before the new parameters take effect. An
initiator MAY perform integrity checking.

An iSCSI initiator or target MAY terminate a negotiation that does
not terminate within an implementation-specific reasonable time or
number of exchanges but SHOULD allow at least six (6) exchanges.

6.2.1. List Negotiations

In list negotiation, the originator sends a list of values (which may
include "None"), in order of preference.

The responding party MUST respond with the same key and the first
value that it supports (and is allowed to use for the specific
originator) selected from the originator list.

The constant "None" MUST always be used to indicate a missing
function. However, "None" is only a valid selection if it is
explicitly proposed. When "None" is proposed as a selection item in
a negotiation for a key, it indicates to the responder that not
supporting any functionality related to that key is legal, and if
"None" is the negotiation result for such a key, it means that key-
specific semantics are not operational for the negotiation scope
(connection or session) of that key.

If an acceptor does not understand any particular value in a list, it
MUST ignore it. If an acceptor does not support, does not
understand, or is not allowed to use any of the proposed options with
a specific originator, it may use the constant "Reject" or terminate
the negotiation. The selection of a value not proposed MUST be
handled by the originator as a protocol error.

6.2.2. Simple-Value Negotiations

For simple-value negotiations, the accepting party MUST answer with
the same key. The value it selects becomes the negotiation result.

Proposing a value not admissible (e.g., not within the specified
bounds) MAY be answered with the constant "Reject"; otherwise, the
acceptor MUST select an admissible value.

The selection, by the acceptor, of a value not admissible under the
selection rules is considered a protocol error. The selection rules
are key-specific.

For a numerical range, the value selected MUST be an integer within
the proposed range or "Reject" (if the range is unacceptable).

For Boolean negotiations (i.e., keys taking the values "Yes" or
"No"), the accepting party MUST answer with the same key and the
result of the negotiation when the received value does not determine
that result by itself. The last value transmitted becomes the
negotiation result. The rules for selecting the value with which to
answer are expressed as Boolean functions of the value received, and
the value that the accepting party would have selected if given a
choice.

Specifically, the two cases in which answers are OPTIONAL are:

The Boolean function is "AND" and the value "No" is received.
The outcome of the negotiation is "No".

The Boolean function is "OR" and the value "Yes" is received.
The outcome of the negotiation is "Yes".

Responses are REQUIRED in all other cases, and the value chosen and
sent by the acceptor becomes the outcome of the negotiation.

6.3. Login Phase

The Login Phase establishes an iSCSI connection between an initiator
and a target; it also creates a new session or associates the
connection to an existing session. The Login Phase sets the iSCSI
protocol parameters and security parameters, and authenticates the
initiator and target to each other.

The Login Phase is only implemented via Login Requests and Responses.
The whole Login Phase is considered as a single task and has a single
Initiator Task Tag (similar to the linked SCSI commands).

There MUST NOT be more than one outstanding Login Request or Login
Response on an iSCSI connection. An outstanding PDU in this context
is one that has not been acknowledged by the remote iSCSI side.

The default MaxRecvDataSegmentLength is used during login.

The Login Phase sequence of requests and responses proceeds as
follows:

Login initial request

- Login partial response (optional)

More Login Requests and Responses (optional)

Login Final-Response (mandatory)

The initial Login Request of any connection MUST include the
InitiatorName key=value pair. The initial Login Request of the first
connection of a session MAY also include the SessionType key=value
pair. For any connection within a session whose type is not
"Discovery", the first Login Request MUST also include the TargetName
key=value pair.

The Login Final-Response accepts or rejects the Login Request.

The Login Phase MAY include a SecurityNegotiation stage and a
LoginOperationalNegotiation stage and MUST include at least one of
them, but the included stage MAY be empty except for the mandatory
names.

The Login Requests and Responses contain a field (CSG) that indicates
the current negotiation stage (SecurityNegotiation or
LoginOperationalNegotiation). If both stages are used, the
SecurityNegotiation MUST precede the LoginOperationalNegotiation.

Some operational parameters can be negotiated outside the login
through Text Requests and Responses.

Authentication-related security keys (Section 12) MUST be completely
negotiated within the Login Phase. The use of underlying IPsec
security is specified in Section 9.3, in [RFC3723], and in [RFC7146].
iSCSI support for security within the protocol only consists of
authentication in the Login Phase.

In some environments, a target or an initiator is not interested in
authenticating its counterpart. It is possible to bypass
authentication through the Login Request and Response.

The initiator and target MAY want to negotiate iSCSI authentication
parameters. Once this negotiation is completed, the channel is
considered secure.

Most of the negotiation keys are only allowed in a specific stage.
The keys used during the SecurityNegotiation stage are listed in
Section 12, and the keys used during the LoginOperationalNegotiation
stage are discussed in Section 13. Only a limited set of keys
(marked as Any-Stage in Section 13) may be used in either of the two
stages.

Any given Login Request or Response belongs to a specific stage; this
determines the negotiation keys allowed with the request or response.
Sending a key that is not allowed in the current stage is considered
a protocol error.

Stage transition is performed through a command exchange
(request/response) that carries the T bit and the same CSG code.
During this exchange, the next stage is selected by the target via
the Next Stage code (NSG). The selected NSG MUST NOT exceed the
value stated by the initiator. The initiator can request a
transition whenever it is ready, but a target can only respond with a
transition after one is proposed by the initiator.

In a negotiation sequence, the T bit settings in one Login Request-
Login Response pair have no bearing on the T bit settings of the next
pair. An initiator that has the T bit set to 1 in one pair and is
answered with a T bit setting of 0 may issue the next request with
the T bit set to 0.

When a transition is requested by the initiator and acknowledged by
the target, both the initiator and target switch to the selected
stage.

Targets MUST NOT submit parameters that require an additional
initiator Login Request in a Login Response with the T bit set to 1.

Stage transitions during login (including entering and exit) are only
possible as outlined in the following table:

The Login Final-Response that accepts a Login Request can only come
as a response to a Login Request with the T bit set to 1, and both
the request and response MUST indicate FullFeaturePhase as the next
phase via the NSG field.

Neither the initiator nor the target should attempt to declare or
negotiate a parameter more than once during login, except for
responses to specific keys that explicitly allow repeated key
declarations (e.g., TargetAddress). An attempt to
renegotiate/redeclare parameters not specifically allowed MUST be
detected by the initiator and target. If such an attempt is detected
by the target, the target MUST respond with a Login reject (initiator
error); if detected by the initiator, the initiator MUST drop the
connection.

6.3.1. Login Phase Start

The Login Phase starts with a Login Request from the initiator to the
target. The initial Login Request includes:

Protocol version supported by the initiator

iSCSI Initiator Name and iSCSI Target Name

- ISID, TSIH, and connection IDs

Negotiation stage that the initiator is ready to enter

A login may create a new session, or it may add a connection to an
existing session. Between a given iSCSI initiator node (selected
only by an InitiatorName) and a given iSCSI target defined by an
iSCSI TargetName and a Target Portal Group Tag, the login results are
defined by the following table:

Login Response with Login reject. This is an immediate
rejection from the target that causes the connection to
terminate and the session to terminate if this is the first (or
only) connection of a new session. The T bit, the CSG field,
and the NSG field are reserved.

Login Response with Login accept as the Final-Response (T bit
set to 1 and the NSG in both request and response is set to
FullFeaturePhase). The response includes the protocol version
supported by the target and the session ID and may include iSCSI
operational or security parameters (that depend on the current
stage).

Login Response with Login accept as a partial response (NSG not
set to FullFeaturePhase in both request and response) that
indicates the start of a negotiation sequence. The response
includes the protocol version supported by the target and either
security or iSCSI parameters (when no security mechanism is
chosen) supported by the target.

If the initiator decides to forego the SecurityNegotiation stage, it
issues the Login with the CSG set to LoginOperationalNegotiation, and
the target may reply with a Login Response that indicates that it is
unwilling to accept the connection (see Section 11.13) without
SecurityNegotiation and will terminate the connection with a response
of Authentication failure (see Section 11.13.5).

If the initiator is willing to negotiate iSCSI security, but is
unwilling to make the initial parameter proposal and may accept a
connection without iSCSI security, it issues the Login with the T bit
set to 1, the CSG set to SecurityNegotiation, and the NSG set to
LoginOperationalNegotiation. If the target is also ready to skip
security, the Login Response only contains the TargetPortalGroupTag
key (see Section 13.9), the T bit set to 1, the CSG set to
SecurityNegotiation, and the NSG set to LoginOperationalNegotiation.

An initiator that chooses to operate without iSCSI security and with
all the operational parameters taking the default values issues the
Login with the T bit set to 1, the CSG set to
LoginOperationalNegotiation, and the NSG set to FullFeaturePhase. If
the target is also ready to forego security and can finish its
LoginOperationalNegotiation, the Login Response has the T bit set to
1, the CSG set to LoginOperationalNegotiation, and the NSG set to
FullFeaturePhase in the next stage.

During the Login Phase, the iSCSI target MUST return the
TargetPortalGroupTag key with the first Login Response PDU with which
it is allowed to do so (i.e., the first Login Response issued after
the first Login Request with the C bit set to 0) for all session
types. The TargetPortalGroupTag key value indicates the iSCSI portal
group servicing the Login Request PDU. If the reconfiguration of
iSCSI portal groups is a concern in a given environment, the iSCSI
initiator should use this key to ascertain that it had indeed
initiated the Login Phase with the intended target portal group.

6.3.2. iSCSI Security Negotiation

The security exchange sets the security mechanism and authenticates
the initiator and the target to each other. The exchange proceeds
according to the authentication method chosen in the negotiation
phase and is conducted using the key=value parameters carried in the
Login Requests and Responses.

An initiator-directed negotiation proceeds as follows:

The initiator sends a Login Request with an ordered list of the
options it supports (authentication algorithm). The options are
listed in the initiator's order of preference. The initiator
MAY also send private or public extension options.

The target MUST reply with the first option in the list it
supports and is allowed to use for the specific initiator,
unless it does not support any, in which case it MUST answer
with "Reject" (see Section 6.2). The parameters are encoded in
UTF-8 as key=value. For security parameters, see Section 12.

When the initiator considers itself ready to conclude the
SecurityNegotiation stage, it sets the T bit to 1 and the NSG to
what it would like the next stage to be. The target will then
set the T bit to 1 and set the NSG to the next stage in the
Login Response when it finishes sending its security keys. The
next stage selected will be the one the target selected. If the
next stage is FullFeaturePhase, the target MUST reply with a
Login Response with the TSIH value.

If the security negotiation fails at the target, then the target MUST
send the appropriate Login Response PDU. If the security negotiation
fails at the initiator, the initiator SHOULD close the connection.

It should be noted that the negotiation might also be directed by the
target if the initiator does support security but is not ready to
direct the negotiation (propose options); see Appendix B for an
example.

6.3.3. Operational Parameter Negotiation during the Login Phase

Operational parameter negotiation during the Login Phase MAY be done:

starting with the first Login Request if the initiator does not
propose any security/integrity option.

starting immediately after the security negotiation if the
initiator and target perform such a negotiation.

Operational parameter negotiation MAY involve several Login Request-
Login Response exchanges started and terminated by the initiator.
The initiator MUST indicate its intent to terminate the negotiation
by setting the T bit to 1; the target sets the T bit to 1 on the last
response.

Even when the initiator indicates its intent to switch stages by
setting the T bit to 1 in a Login Request, the target MAY respond
with a Login Response with the T bit set to 0. In that case, the
initiator SHOULD continue to set the T bit to 1 in subsequent Login
Requests (even empty requests) that it sends, until the target sends
a Login Response with the T bit set to 1 or sends a key that requires
the initiator to set the T bit to 0.

Some session-specific parameters can only be specified during the
Login Phase of the first connection of a session (i.e., begun by a
Login Request that contains a zero-valued TSIH) -- the leading Login
Phase (e.g., the maximum number of connections that can be used for
this session).

A session is operational once it has at least one connection in the
Full Feature Phase. New or replacement connections can only be added
to a session after the session is operational.

For operational parameters, see Section 13.

6.3.4. Connection Reinstatement

Connection reinstatement is the process of an initiator logging in
with an ISID-TSIH-CID combination that is possibly active from the
target's perspective, which causes the implicit logging out of the
connection corresponding to the CID and reinstatement of a new Full
Feature Phase iSCSI connection in its place (with the same CID).
Thus, the TSIH in the Login Request PDU MUST be non-zero, and the CID
does not change during a connection reinstatement. The Login Request
performs the logout function of the old connection if an explicit
logout was not performed earlier. In sessions with a single
connection, this may imply the opening of a second connection with
the sole purpose of cleaning up the first. Targets MUST support
opening a second connection even when they do not support multiple
connections in the Full Feature Phase if ErrorRecoveryLevel is 2 and
SHOULD support opening a second connection if ErrorRecoveryLevel is
less than 2.

If the operational ErrorRecoveryLevel is 2, connection reinstatement
enables future task reassignment. If the operational
ErrorRecoveryLevel is less than 2, connection reinstatement is the
replacement of the old CID without enabling task reassignment. In
this case, all the tasks that were active on the old CID must be
immediately terminated without further notice to the initiator.

The initiator connection state MUST be CLEANUP_WAIT (Section 8.1.3)
when the initiator attempts a connection reinstatement.

In practical terms, in addition to the implicit logout of the old
connection, reinstatement is equivalent to a new connection login.

6.3.5. Session Reinstatement, Closure, and Timeout

Session reinstatement is the process of an initiator logging in with
an ISID that is possibly active from the target's perspective for
that initiator, thus implicitly logging out the session that
corresponds to the ISID and reinstating a new iSCSI session in its
place (with the same ISID). Therefore, the TSIH in the Login PDU
MUST be zero to signal session reinstatement. Session reinstatement
causes all the tasks that were active on the old session to be
immediately terminated by the target without further notice to the
initiator.

The initiator session state MUST be FAILED (Section 8.3) when the
initiator attempts a session reinstatement.

Session closure is an event defined to be one of the following:

a successful "session close" logout.

a successful "connection close" logout for the last Full Feature
Phase connection when no other connection in the session is
waiting for cleanup (Section 8.2) and no tasks in the session
are waiting for reassignment.

Session timeout is an event defined to occur when the last connection
state timeout expires and no tasks are waiting for reassignment.
This takes the session to the FREE state (see the session state
diagrams in Section 8.3).

6.3.5.1. Loss of Nexus Notification

The iSCSI layer provides the SCSI layer with the "I_T nexus loss"
notification when any one of the following events happens:

successful completion of session reinstatement

session closure event

session timeout event

Certain SCSI object clearing actions may result due to the
notification in the SCSI end nodes, as documented in Appendix E.

6.3.6. Session Continuation and Failure

Session continuation is the process by which the state of a
preexisting session continues to be used by connection reinstatement
(Section 6.3.4) or by adding a connection with a new CID. Either of
these actions associates the new transport connection with the
session state.

Session failure is an event where the last Full Feature Phase
connection reaches the CLEANUP_WAIT state (Section 8.2) or completes
a successful recovery logout, thus causing all active tasks (that are
formerly allegiant to the connection) to start waiting for task
reassignment.

6.4. Operational Parameter Negotiation outside the Login Phase

Some operational parameters MAY be negotiated outside (after) the
Login Phase.

Parameter negotiation in the Full Feature Phase is done through Text
Requests and Responses. Operational parameter negotiation MAY
involve several Text Request-Text Response exchanges, all of which
use the same Initiator Task Tag; the initiator always starts and
terminates each of these exchanges. The initiator MUST indicate its
intent to finish the negotiation by setting the F bit to 1; the
target sets the F bit to 1 on the last response.

If the target responds to a Text Request with the F bit set to 1 with
a Text Response with the F bit set to 0, the initiator should keep
sending the Text Request (even empty requests) with the F bit set to
1 while it still wants to finish the negotiation, until it receives
the Text Response with the F bit set to 1. Responding to a Text
Request with the F bit set to 1 with an empty (no key=value pairs)
response with the F bit set to 0 is discouraged.

Even when the initiator indicates its intent to finish the
negotiation by setting the F bit to 1 in a Text Request, the target
MAY respond with a Text Response with the F bit set to 0. In that
case, the initiator SHOULD continue to set the F bit to 1 in
subsequent Text Requests (even empty requests) that it sends, until
the target sends the final Text Response with the F bit set to 1.
Note that in the same case of a Text Request with the F bit set to 1,
the target SHOULD NOT respond with an empty (no key=value pairs) Text
Response with the F bit set to 0, because such a response may cause
the initiator to abandon the negotiation.

Targets MUST NOT submit parameters that require an additional
initiator Text Request in a Text Response with the F bit set to 1.

In a negotiation sequence, the F bit settings in one Text Request-
Text Response pair have no bearing on the F bit settings of the next
pair. An initiator that has the F bit set to 1 in a request and is
being answered with an F bit setting of 0 may issue the next request
with the F bit set to 0.

Whenever the target responds with the F bit set to 0, it MUST set the
Target Transfer Tag to a value other than the default 0xffffffff.

An initiator MAY reset an operational parameter negotiation by
issuing a Text Request with the Target Transfer Tag set to the value
0xffffffff after receiving a response with the Target Transfer Tag
set to a value other than 0xffffffff. A target may reset an
operational parameter negotiation by answering a Text Request with a
Reject PDU.

Neither the initiator nor the target should attempt to declare or
negotiate a parameter more than once during any negotiation sequence,
except for responses to specific keys that explicitly allow repeated
key declarations (e.g., TargetAddress). If such an attempt is
detected by the target, the target MUST respond with a Reject PDU
with a reason of "Protocol Error". The initiator MUST reset the
negotiation as outlined above.

Parameters negotiated by a text exchange negotiation sequence only
become effective after the negotiation sequence is completed.

7. iSCSI Error Handling and Recovery

7.1. Overview

7.1.1. Background

The following two considerations prompted the design of much of the
error recovery functionality in iSCSI:

An iSCSI PDU may fail the digest check and be dropped, despite
being received by the TCP layer. The iSCSI layer must
optionally be allowed to recover such dropped PDUs.

A TCP connection may fail at any time during the data transfer.
All the active tasks must optionally be allowed to be continued
on a different TCP connection within the same session.

Implementations have considerable flexibility in deciding what degree
of error recovery to support, when to use it, and by which mechanisms
to achieve the required behavior. Only the externally visible
actions of the error recovery mechanisms must be standardized to
ensure interoperability.

This section describes a general model for recovery in support of
interoperability. See Appendix D for further details on how the
described model may be implemented. Compliant implementations do not
have to match the implementation details of this model as presented,
but the external behavior of such implementations must correspond to
the externally observable characteristics of the presented model.

7.1.2. Goals

The major design goals of the iSCSI error recovery scheme are as
follows:

Allow iSCSI implementations to meet different requirements by
defining a collection of error recovery mechanisms from which
implementations may choose.

Ensure interoperability between any two implementations
supporting different sets of error recovery capabilities.

Define the error recovery mechanisms to ensure command ordering
even in the face of errors, for initiators that demand ordering.

Do not make additions in the fast path, but allow moderate
complexity in the error recovery path.

Prevent both the initiator and target from attempting to recover
the same set of PDUs at the same time. For example, there must
be a clear "error recovery functionality distribution" between
the initiator and target.

7.1.3. Protocol Features and State Expectations

The initiator mechanisms defined in connection with error recovery
are:

a) NOP-Out to probe sequence numbers of the target (Section 11.18)

b) Command retry (Section 7.2.1)

c) Recovery R2T support (Section 7.8)

d) Requesting retransmission of status/data/R2T using the SNACK

facility (Section 11.16)

e) Acknowledging the receipt of the data (Section 11.16)

f) Reassigning the connection allegiance of a task to a different

TCP connection (Section 7.2.2)

g) Terminating the entire iSCSI session to start afresh

(Section 7.1.4.4)

The target mechanisms defined in connection with error recovery are:

a) NOP-In to probe sequence numbers of the initiator

(Section 11.19)

b) Requesting retransmission of data using the recovery R2T

feature (Section 7.8)
c) SNACK support (Section 11.16)

d) Requesting that parts of read data be acknowledged

(Section 11.7.2)
e) Allegiance reassignment support (Section 7.2.2)

f) Terminating the entire iSCSI session to force the initiator to

start over (Section 7.1.4.4)

For any outstanding SCSI command, it is assumed that iSCSI, in
conjunction with SCSI at the initiator, is able to keep enough
information to be able to rebuild the command PDU and that outgoing
data is available (in host memory) for retransmission while the
command is outstanding. It is also assumed that at the target,
incoming data (read data) MAY be kept for recovery, or it can be
reread from a device server.

It is further assumed that a target will keep the "status and sense"
for a command it has executed if it supports status retransmission.

A target that agrees to support data retransmission is expected to be
prepared to retransmit the outgoing data (i.e., Data-In) on request
until either the status for the completed command is acknowledged or
the data in question has been separately acknowledged.

7.1.4. Recovery Classes

iSCSI enables the following classes of recovery (in the order of
increasing scope of affected iSCSI tasks):

- within a command (i.e., without requiring command restart)
- within a connection (i.e., without requiring the connection to
be rebuilt, but perhaps requiring command restart)
- connection recovery (i.e., perhaps requiring connections to be
rebuilt and commands to be reissued)

session recovery

The recovery scenarios detailed in the rest of this section are
representative rather than exclusive. In every case, they detail the
lowest recovery class that MAY be attempted. The implementer is left
to decide under which circumstances to escalate to the next recovery
class and/or what recovery classes to implement. Both the iSCSI
target and initiator MAY escalate the error handling to an error
recovery class, which impacts a larger number of iSCSI tasks in any
of the cases identified in the following discussion.

In all classes, the implementer has the choice of deferring errors to
the SCSI initiator (with an appropriate response code), in which case
the task, if any, has to be removed from the target and all the side
effects, such as ACA, must be considered.

The use of within-connection and within-command recovery classes MUST
NOT be attempted before the connection is in the Full Feature Phase.

In the detailed description of the recovery classes, the mandating
terms (MUST, SHOULD, MAY, etc.) indicate normative actions to be
executed if the recovery class is supported (see Section 7.1.5 for
the related negotiation semantics) and used.

7.1.4.1. Recovery Within-command

At the target, the following cases lend themselves to within-command
recovery:

Lost data PDU - realized through one of the following:

a) Data digest error - dealt with as specified in Section 7.8,

using the option of a recovery R2T

b) Sequence reception timeout (no data or partial-data-and-no-
F-bit) - considered an implicit sequence error and dealt with
as specified in Section 7.9, using the option of a recovery R2T

c) Header digest error, which manifests as a sequence reception

timeout or a sequence error - dealt with as specified in
Section 7.9, using the option of a recovery R2T

At the initiator, the following cases lend themselves to within-
command recovery:

Lost data PDU or lost R2T - realized through one of the following:

a) Data digest error - dealt with as specified in Section 7.8,

using the option of a SNACK

b) Sequence reception timeout (no status) or response reception

timeout - dealt with as specified in Section 7.9, using the
option of a SNACK

c) Header digest error, which manifests as a sequence reception

timeout or a sequence error - dealt with as specified in
Section 7.9, using the option of a SNACK

To avoid a race with the target, which may already have a recovery
R2T or a termination response on its way, an initiator SHOULD NOT
originate a SNACK for an R2T based on its internal timeouts (if any).
Recovery in this case is better left to the target.

The timeout values used by the initiator and target are outside the
scope of this document. A sequence reception timeout is generally a
large enough value to allow the data sequence transfer to be
complete.

7.1.4.2. Recovery Within-connection

At the initiator, the following cases lend themselves to within-
connection recovery:

a) Requests not acknowledged for a long time. Requests are

acknowledged explicitly through the ExpCmdSN or implicitly by

receiving data and/or status. The initiator MAY retry

non-acknowledged commands as specified in Section 7.2.

b) Lost iSCSI numbered response. It is recognized by either

identifying a data digest error on a Response PDU or a Data-In
PDU carrying the status, or receiving a Response PDU with a
higher StatSN than expected. In the first case, digest error
handling is done as specified in Section 7.8, using the option
of a SNACK. In the second case, sequence error handling is
done as specified in Section 7.9, using the option of a SNACK.

At the target, the following cases lend themselves to within-
connection recovery:

Status/Response not acknowledged for a long time. The target
MAY issue a NOP-In (with a valid Target Transfer Tag or
otherwise) that carries the next status sequence number it is
going to use in the StatSN field. This helps the initiator
detect any missing StatSN(s) and issue a SNACK for the status.

The timeout values used by the initiator and the target are outside
the scope of this document.

7.1.4.3. Connection Recovery

At an iSCSI initiator, the following cases lend themselves to
connection recovery:

a) TCP connection failure: The initiator MUST close the

connection. It then MUST either implicitly or explicitly log
out the failed connection with the reason code "remove the
connection for recovery" and reassign connection allegiance for
all commands still in progress associated with the failed
connection on one or more connections (some or all of which MAY
be newly established connections) using the "TASK REASSIGN"
task management function (see Section 11.5.1). For an
initiator, a command is in progress as long as it has not
received a response or a Data-In PDU including status.

Note: The logout function is mandatory. However, a new

connection establishment is only mandatory if the failed

connection was the last or only connection in the session.

b) Receiving an Asynchronous Message that indicates that one or

all connections in a session have been dropped. The initiator
MUST handle it as a TCP connection failure for the
connection(s) referred to in the message.

At an iSCSI target, the following cases lend themselves to connection
recovery:

TCP connection failure: The target MUST close the connection
and, if more than one connection is available, the target SHOULD
send an Asynchronous Message that indicates that it has dropped
the connection. Then, the target will wait for the initiator to
continue recovery.

7.1.4.4. Session Recovery

Session recovery should be performed when all other recovery attempts
have failed. Very simple initiators and targets MAY perform session
recovery on all iSCSI errors and rely on recovery on the SCSI layer
and above.

Session recovery implies the closing of all TCP connections,
internally aborting all executing and queued tasks for the given
initiator at the target, terminating all outstanding SCSI commands
with an appropriate SCSI service response at the initiator, and
restarting a session on a new set of connection(s) (TCP connection
establishment and login on all new connections).

For possible clearing effects of session recovery on SCSI and iSCSI
objects, refer to Appendix E.

7.1.5. Error Recovery Hierarchy

The error recovery classes described so far are organized into a
hierarchy for ease in understanding and to limit the complexity of
the implementation. With a few well-defined recovery levels,
interoperability is easier to achieve. The attributes of this
hierarchy are as follows:

a) Each level is a superset of the capabilities of the previous

level. For example, Level 1 support implies supporting all
capabilities of Level 0 and more.

b) As a corollary, supporting a higher error recovery level means

increased sophistication and possibly an increase in resource
requirements.

When a defined value of ErrorRecoveryLevel is proposed by an
originator in a text negotiation, the originator MUST support the
functionality defined for the proposed value and, additionally,
functionality corresponding to any defined value numerically less
than the proposed value. When a defined value of ErrorRecoveryLevel
is returned by a responder in a text negotiation, the responder MUST
support the functionality corresponding to the ErrorRecoveryLevel it
is accepting.

When either party attempts to use error recovery functionality beyond
what is negotiated, the recovery attempts MAY fail, unless an
a priori agreement outside the scope of this document exists between
the two parties to provide such support.

Implementations MUST support error recovery level "0", while the rest
are OPTIONAL to implement. In implementation terms, the above
striation means that the following incremental sophistication with
each level is required:

7.2. Retry and Reassign in Recovery

This section summarizes two important and somewhat related iSCSI
protocol features used in error recovery.

7.2.1. Usage of Retry

By resending the same iSCSI Command PDU ("retry") in the absence of a
command acknowledgment (by way of an ExpCmdSN update) or a response,
an initiator attempts to "plug" (what it thinks are) the
discontinuities in CmdSN ordering on the target end. Discarded
command PDUs, due to digest errors, may have created these
discontinuities.

Retry MUST NOT be used for reasons other than plugging command
sequence gaps and, in particular, cannot be used for requesting PDU
retransmissions from a target. Any such PDU retransmission requests
for a currently allegiant command in progress may be made using the
SNACK mechanism described in Section 11.16, although the usage of
SNACK is OPTIONAL.

If initiators, as part of plugging command sequence gaps as described
above, inadvertently issue retries for allegiant commands already in
progress (i.e., targets did not see the discontinuities in CmdSN
ordering), the duplicate commands are silently ignored by targets as
specified in Section 4.2.2.1.

When an iSCSI command is retried, the command PDU MUST carry the
original Initiator Task Tag and the original operational attributes
(e.g., flags, function names, LUN, CDB, etc.) as well as the original
CmdSN. The command being retried MUST be sent on the same connection
as the original command, unless the original connection was already
successfully logged out.

7.2.2. Allegiance Reassignment

By issuing a "TASK REASSIGN" task management request
(Section 11.5.1), the initiator signals its intent to continue an
already active command (but with no current connection allegiance) as
part of connection recovery. This means that a new connection
allegiance is requested for the command, which seeks to associate it
to the connection on which the task management request is being
issued. Before the allegiance reassignment is attempted for a task,
an implicit or explicit Logout with the reason code "remove the
connection for recovery" (see Section 11.14.1) MUST be successfully
completed for the previous connection to which the task was
allegiant.

In reassigning connection allegiance for a command, the target SHOULD
continue the command from its current state. For example, when
reassigning read commands, the target SHOULD take advantage of the
ExpDataSN field provided by the Task Management Function Request
(which must be set to 0 if there was no data transfer) and bring the
read command to completion by sending the remaining data and sending
(or resending) the status. The ExpDataSN acknowledges all data sent
up to, but not including, the Data-In PDU and/or R2T with the DataSN
(or R2TSN) equal to the ExpDataSN. However, targets may choose to
send/receive all unacknowledged data or all of the data on a
reassignment of connection allegiance if unable to recover or
maintain accurate state. Initiators MUST NOT subsequently request
data retransmission through Data SNACK for PDUs numbered less than
the ExpDataSN (i.e., prior to the acknowledged sequence number). For
all types of commands, a reassignment request implies that the task
is still considered in progress by the initiator, and the target must
conclude the task appropriately if the target returns the "Function
complete" response to the reassignment request. This might possibly
involve retransmission of data/R2T/status PDUs as necessary but MUST
involve the (re)transmission of the status PDU.

It is OPTIONAL for targets to support the allegiance reassignment.
This capability is negotiated via the ErrorRecoveryLevel text key
during the login time. When a target does not support allegiance
reassignment, it MUST respond with a task management response code of
"Task allegiance reassignment not supported". If allegiance
reassignment is supported by the target but the task is still
allegiant to a different connection, or a successful recovery Logout
of the previously allegiant connection was not performed, the target
MUST respond with a task management response code of "Task still
allegiant".

If allegiance reassignment is supported by the target, the task
management response to the reassignment request MUST be issued before
the reassignment becomes effective.

If a SCSI command that involves data input is reassigned, any SNACK
Tag it holds for a final response from the original connection is
deleted, and the default value of 0 MUST be used instead.

7.3. Usage of Reject PDU in Recovery

Targets MUST NOT implicitly terminate an active task by sending a
Reject PDU for any PDU exchanged during the life of the task. If the
target decides to terminate the task, a Response PDU (SCSI, Text,
Task, etc.) must be returned by the target to conclude the task. If
the task had never been active before the Reject (i.e., the Reject is
on the command PDU), targets should not send any further responses
because the command itself is being discarded.

The above rule means that the initiator can eventually expect a
response on receiving Rejects, if the received Reject is for a PDU
other than the command PDU itself. The non-command Rejects only have
diagnostic value in logging the errors, and they can be used for
retransmission decisions by the initiators.

The CmdSN of the rejected command PDU (if it is a non-immediate
command) MUST NOT be considered received by the target (i.e., a
command sequence gap must be assumed for the CmdSN), even though the
CmdSN of the rejected command PDU may be reliably ascertained. Upon
receiving the Reject, the initiator MUST plug the CmdSN gap in order
to continue to use the session. The gap may be plugged by either
transmitting a command PDU with the same CmdSN or aborting the task
(see Section 7.11 for information regarding how an abort may plug a
CmdSN gap).

When a data PDU is rejected and its DataSN can be ascertained, a
target MUST advance the ExpDataSN for the current data burst if a
recovery R2T is being generated. The target MAY advance its
ExpDataSN if it does not attempt to recover the lost data PDU.

7.4. Error Recovery Considerations for Discovery Sessions

7.4.1. ErrorRecoveryLevel for Discovery Sessions

The negotiation of the key ErrorRecoveryLevel is not required for
Discovery sessions -- i.e., for sessions that negotiated
"SessionType=Discovery" -- because the default value of 0 is
necessary and sufficient for Discovery sessions. It is, however,
possible that some legacy iSCSI implementations might attempt to
negotiate the ErrorRecoveryLevel key on Discovery sessions. When
such a negotiation attempt is made by the remote side, a compliant
iSCSI implementation MUST propose a value of 0 (zero) in response.
The operational ErrorRecoveryLevel for Discovery sessions thus MUST
be 0. This naturally follows from the functionality constraints that
Section 4.3 imposes on Discovery sessions.

7.4.2. Reinstatement Semantics for Discovery Sessions

Discovery sessions are intended to be relatively short-lived.
Initiators are not expected to establish multiple Discovery sessions
to the same iSCSI Network Portal. An initiator may use the same
iSCSI Initiator Name and ISID when establishing different unique
sessions with different targets and/or different portal groups. This
behavior is discussed in Section 10.1.1 and is, in fact, encouraged
as conservative reuse of ISIDs.

The ISID RULE in Section 4.4.3 states that there must not be more
than one session with a matching 4-tuple: <InitiatorName, ISID,
TargetName, TargetPortalGroupTag>. While the spirit of the ISID RULE
applies to Discovery sessions the same as it does for Normal
sessions, note that some Discovery sessions differ from the Normal
sessions in two important aspects:

a) Because Appendix C allows a Discovery session to be established

without specifying a TargetName key in the Login Request PDU
(let us call such a session an "Unnamed" Discovery session),
there is no target node context to enforce the ISID RULE.

b) Portal groups are defined only in the context of a target node.

When the TargetName key is NULL-valued (i.e., not specified),
the TargetPortalGroupTag thus cannot be ascertained to enforce
the ISID RULE.

The following two sections describe Unnamed Discovery sessions and
Named Discovery sessions, respectively.

7.4.2.1. Unnamed Discovery Sessions

For Unnamed Discovery sessions, neither the TargetName nor the
TargetPortalGroupTag is available to the targets in order to enforce
the ISID RULE. Therefore, the following rule applies.

UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
following 4-tuple for Unnamed Discovery sessions: <InitiatorName,
ISID, NULL, TargetAddress>. The following semantics are implied by
this uniqueness requirement.

Targets SHOULD allow concurrent establishment of one Discovery
session with each of its Network Portals by the same initiator port
with a given iSCSI Node Name and an ISID. Each of the concurrent
Discovery sessions, if established by the same initiator port to
other Network Portals, MUST be treated as independent sessions --
i.e., one session MUST NOT reinstate the other.

A new Unnamed Discovery session that has a matching <InitiatorName,
ISID, NULL, TargetAddress> to an existing Discovery session MUST
reinstate the existing Unnamed Discovery session. Note thus that
only an Unnamed Discovery session may reinstate another Unnamed
Discovery session.

7.4.2.2. Named Discovery Sessions

For Named Discovery sessions, the TargetName key is specified by the
initiator, and thus the target can unambiguously ascertain the
TargetPortalGroupTag as well. Since all the four elements of the
4-tuple are known, the ISID RULE MUST be enforced by targets with no
changes from Section 4.4.3 semantics. A new session with a matching
<InitiatorName, ISID, TargetName, TargetPortalGroupTag> thus will
reinstate an existing session. Note in this case that any new iSCSI
session (Discovery or Normal) with the matching 4-tuple may reinstate
an existing Named Discovery iSCSI session.

7.4.3. Target PDUs during Discovery

Targets SHOULD NOT send any responses other than a Text Response and
Logout Response on a Discovery session, once in the Full Feature
Phase.

Implementation Note: A target may simply drop the connection in a
Discovery session when it would have requested a Logout via an Async
Message on Normal sessions.

7.5. Connection Timeout Management

iSCSI defines two session-global timeout values (in seconds) --
Time2Wait and Time2Retain -- that are applicable when an iSCSI Full
Feature Phase connection is taken out of service either intentionally
or by an exception. Time2Wait is the initial "respite time" before
attempting an explicit/implicit Logout for the CID in question or
task reassignment for the affected tasks (if any). Time2Retain is
the maximum time after the initial respite interval that the task
and/or connection state(s) is/are guaranteed to be maintained on the
target to cater to a possible recovery attempt. Recovery attempts
for the connection and/or task(s) SHOULD NOT be made before
Time2Wait seconds but MUST be completed within Time2Retain seconds
after that initial Time2Wait waiting period.

7.5.1. Timeouts on Transport Exception Events

A transport connection shutdown or a transport reset without any
preceding iSCSI protocol interactions informing the endpoints of the
fact causes a Full Feature Phase iSCSI connection to be abruptly
terminated. The timeout values to be used in this case are the
negotiated values of DefaultTime2Wait (Section 13.15) and
DefaultTime2Retain (Section 13.16) text keys for the session.

7.5.2. Timeouts on Planned Decommissioning

Any planned decommissioning of a Full Feature Phase iSCSI connection
is preceded by either a Logout Response PDU or an Async Message PDU.
The Time2Wait and Time2Retain field values (Section 11.15) in a
Logout Response PDU, and the Parameter2 and Parameter3 fields of an
Async Message (AsyncEvent types "drop the connection" or "drop all
the connections"; see Section 11.9.1), specify the timeout values to
be used in each of these cases.

These timeout values are only applicable for the affected connection
and the tasks active on that connection. These timeout values have
no bearing on initiator timers (if any) that are already running on
connections or tasks associated with that session.

7.6. Implicit Termination of Tasks

A target implicitly terminates the active tasks due to iSCSI protocol
dynamics in the following cases:

a) When a connection is implicitly or explicitly logged out with

the reason code "close the connection" and there are active
tasks allegiant to that connection.

b) When a connection fails and eventually the connection state

times out (state transition M1 in Section 8.2.2), and there are
active tasks allegiant to that connection.

c) When a successful Logout with the reason code "remove the

connection for recovery" is performed while there are active
tasks allegiant to that connection, and those tasks eventually
time out after the Time2Wait and Time2Retain periods without
allegiance reassignment.

d) When a connection is implicitly or explicitly logged out with

the reason code "close the session" and there are active tasks
in that session.

If the tasks terminated in cases a), b), c), and d) above are SCSI
tasks, they must be internally terminated as if with CHECK CONDITION
status. This status is only meaningful for appropriately handling
the internal SCSI state and SCSI side effects with respect to
ordering, because this status is never communicated back as a
terminating status to the initiator. However, additional actions may
have to be taken at the SCSI level, depending on the SCSI context as
defined by the SCSI standards (e.g., queued commands and ACA; UA for
the next command on the I_T nexus in cases a), b), and c); etc. --
see [SAM2] and [SPC3]).

7.7. Format Errors

The following two explicit violations of PDU layout rules are format
errors:

a) Illegal contents of any PDU header field except the Opcode

(legal values are specified in Section 11).

b) Inconsistent field contents (consistent field contents are

specified in Section 11).

Format errors indicate a major implementation flaw in one of the
parties.

When a target or an initiator receives an iSCSI PDU with a format
error, it MUST immediately terminate all transport connections in the
session with either a connection close or a connection reset, and
escalate the format error to session recovery (see Section 7.1.4.4).

All initiator-detected PDU construction errors MUST be considered as
format errors. Some examples of such errors are:

NOP-In with a valid TTT but an invalid LUN

- NOP-In with a valid ITT (i.e., a NOP-In response) and also a
valid TTT

7.8. Digest Errors

The discussion below regarding the legal choices in handling digest
errors excludes session recovery as an explicit option, but either
party detecting a digest error may choose to escalate the error to
session recovery.

When a target or an initiator receives any iSCSI PDU with a header
digest error, it MUST either discard the header and all data up to
the beginning of a later PDU or close the connection. Because the
digest error indicates that the length field of the header may have
been corrupted, the location of the beginning of a later PDU needs to
be reliably ascertained by other means, such as the operation of a
Sync and Steering layer.

When a target receives any iSCSI PDU with a payload digest error, it
MUST answer with a Reject PDU with a reason code of Data-Digest-Error
and discard the PDU.

If the discarded PDU is a solicited or unsolicited iSCSI data PDU
(for immediate data in a command PDU, the non-data PDU rule below
applies), the target MUST do one of the following:
a) Request retransmission with a recovery R2T.

b) Terminate the task with a SCSI Response PDU with a CHECK

CONDITION Status and an iSCSI Condition of "Protocol Service CRC
error" (Section 11.4.7.2). If the target chooses to implement
this option, it MUST wait to receive all the data (signaled by a
data PDU with the Final bit set for all outstanding R2Ts) before
sending the SCSI Response PDU. A task management command (such
as an ABORT TASK) from the initiator during this wait may also
conclude the task.

No further action is necessary for targets if the discarded PDU is
a non-data PDU. In the case of immediate data being present on a
discarded command, the immediate data is implicitly recovered when
the task is retried (see Section 7.2.1), followed by the entire
data transfer for the task.

When an initiator receives any iSCSI PDU with a payload digest error,
it MUST discard the PDU.

If the discarded PDU is an iSCSI data PDU, the initiator MUST do
one of the following:

a) Request the desired data PDU through SNACK. In response to

the SNACK, the target MUST either resend the data PDU or
reject the SNACK with a Reject PDU with a reason code of
"SNACK reject", in which case:

a.1) If the status has not already been sent for the command,

the target MUST terminate the command with a CHECK
CONDITION Status and an iSCSI Condition of "SNACK
rejected" (Section 11.4.7.2).

a.2) If the status was already sent, no further action is

necessary for the target. The initiator in this case
MUST wait for the status to be received and then discard
it, so as to internally signal the completion with CHECK
CONDITION Status and an iSCSI Condition of "Protocol
Service CRC error" (Section 11.4.7.2).

b) Abort the task and terminate the command with an error.

If the discarded PDU is a response PDU or an unsolicited PDU
(e.g., Async, Reject), the initiator MUST do one of the
following:

a) Request PDU retransmission with a status of SNACK.

b) Log out the connection for recovery, and continue the tasks

on a different connection instance as described in
Section 7.2.

c) Log out to close the connection (abort all the commands

associated with the connection).

Note that an unsolicited PDU carries the next StatSN value on an
iSCSI connection, thereby advancing the StatSN. When an initiator
discards one of these PDUs due to a payload digest error, the
entire PDU, including the header, MUST be discarded.
Consequently, the initiator MUST treat the exception like a loss
of any other solicited response PDU.

7.9. Sequence Errors

When an initiator receives an iSCSI R2T/data PDU with an out-of-order
R2TSN/DataSN or a SCSI Response PDU with an ExpDataSN that implies
missing data PDU(s), it means that the initiator must have detected a
header or payload digest error on one or more earlier R2T/data PDUs.

The initiator MUST address these implied digest errors as described
in Section 7.8. When a target receives a data PDU with an out-of-
order DataSN, it means that the target must have hit a header or
payload digest error on at least one of the earlier data PDUs. The
target MUST address these implied digest errors as described in
Section 7.8.

When an initiator receives an iSCSI status PDU with an out-of-order
StatSN that implies missing responses, it MUST address the one or
more missing status PDUs as described in Section 7.8. As a side
effect of receiving the missing responses, the initiator may discover
missing data PDUs. If the initiator wants to recover the missing
data for a command, it MUST NOT acknowledge the received responses
that start from the StatSN of the relevant command until it has
completed receiving all the data PDUs of the command.

When an initiator receives duplicate R2TSNs (due to proactive
retransmission of R2Ts by the target) or duplicate DataSNs (due to
proactive SNACKs by the initiator), it MUST discard the duplicates.

7.10. Message Error Checking

In iSCSI implementations to date, there has been some uncertainty
regarding the extent to which incoming messages have to be checked
for protocol errors, beyond what is strictly required for processing
the inbound message. This section addresses this question.

Unless this document requires it, an iSCSI implementation is not
required to do an exhaustive protocol conformance check on an
incoming iSCSI PDU. The iSCSI implementation in particular is not
required to double-check the remote iSCSI implementation's
conformance to protocol requirements.

7.11. SCSI Timeouts

An iSCSI initiator MAY attempt to plug a command sequence gap on the
target end (in the absence of an acknowledgment of the command by way
of the ExpCmdSN) before the ULP timeout by retrying the
unacknowledged command, as described in Section 7.2.

On a ULP timeout for a command (that carried a CmdSN of n), if the
iSCSI initiator intends to continue the session it MUST abort the
command by using either an appropriate Task Management Function
Request for the specific command or a "close the connection" logout.

When using an ABORT TASK, if the ExpCmdSN is still less than (n + 1),
the target may see the abort request while missing the original
command itself, due to one of the following reasons:

The original command was dropped due to digest error.

The connection on which the original command was sent was
successfully logged out. On logout, the unacknowledged commands
issued on the connection being logged out are discarded.

If the abort request is received and the original command is missing,
targets MUST consider the original command with that RefCmdSN as
received and issue a task management response with the response code
"Function complete". This response concludes the task on both ends.
If the abort request is received and the target can determine (based
on the Referenced Task Tag) that the command was received and
executed, and also that the response was sent prior to the abort,
then the target MUST respond with the response code "Task Does Not
Exist".

7.12. Negotiation Failures

Text Request and Response sequences, when used to set/negotiate
operational parameters, constitute the negotiation/parameter setting.
A negotiation failure is considered to be one or more of the
following:

For a negotiated key, none of the choices are acceptable to one
of the sides in the negotiation.

For a declarative key, the declared value is not acceptable to
the other side in the negotiation.

The Text Request timed out and possibly terminated.

The Text Request was answered with a Reject PDU.

The following two rules should be used to address negotiation
failures:

a) During login, any failure in negotiation MUST be considered a

login process failure; the Login Phase, along with the
connection, MUST be terminated. If the target detects the
failure, it must terminate the login with the appropriate Login
response code.

b) A failure in negotiation during the Full Feature Phase will

terminate the entire negotiation sequence, which may consist of
a series of Text Requests that use the same Initiator Task Tag.
The operational parameters of the session or the connection
MUST continue to be the values agreed upon during an earlier
successful negotiation (i.e., any partial results of this
unsuccessful negotiation MUST NOT take effect and MUST be
discarded).

7.13. Protocol Errors

Mapping framed messages over a "streaming" connection such as TCP
makes the proposed mechanisms vulnerable to simple software framing
errors. On the other hand, the introduction of framing mechanisms to
limit the effects of these errors may be onerous on performance for
simple implementations. Command sequence numbers and the mechanisms
for dropping and reestablishing connections (discussed earlier in
Section 7 and its subsections) help handle this type of mapping
errors.

All violations of iSCSI PDU exchange sequences specified in this
document are also protocol errors. This category of errors can only
be addressed by fixing the implementations; iSCSI defines Reject and
response codes to enable this.

7.14. Connection Failures

iSCSI can keep a session in operation if it is able to keep/establish
at least one TCP connection between the initiator and the target in a
timely fashion. Targets and/or initiators may recognize a failing
connection by either transport-level means (TCP), a gap in the
command sequence number, a response stream that is not filled for a
long time, or a failing iSCSI NOP (acting as a ping). The latter MAY
be used periodically to increase the speed and likelihood of
detecting connection failures. As an example for transport-level
means, initiators and targets MAY also use the keep-alive option (see
[RFC1122]) on the TCP connection to enable early link failure
detection on otherwise idle links.

On connection failure, the initiator and target MUST do one of the
following:

Either side may choose to escalate to session recovery (via the
initiator dropping all the connections or via an Async Message that
announces the similar intent from a target), and the other side MUST
give it precedence. On a connection failure, a target MUST terminate
and/or discard all of the active immediate commands, regardless of
which of the above options is used (i.e., immediate commands are not
recoverable across connection failures).

7.15. Session Errors

If all of the connections of a session fail and cannot be
reestablished in a short time, or if initiators detect protocol
errors repeatedly, an initiator may choose to terminate a session and
establish a new session.

In this case, the initiator takes the following actions:

Resets or closes all the transport connections.

Terminates all outstanding requests with an appropriate response
before initiating a new session. If the same I_T nexus is
intended to be reestablished, the initiator MUST employ session
reinstatement (see Section 6.3.5).

When the session timeout (the connection state timeout for the last
failed connection) happens on the target, it takes the following
actions:

Resets or closes the TCP connections (closes the session).

Terminates all active tasks that were allegiant to the
connection(s) that constituted the session.

A target MUST also be prepared to handle a session reinstatement
request from the initiator that may be addressing session errors.

8. State Transitions

iSCSI connections and iSCSI sessions go through several well-defined
states from the time they are created to the time they are cleared.

The connection state transitions are described in two separate but
dependent sets of state diagrams for ease in understanding. The
first set of diagrams, "standard connection state diagrams",
describes the connection state transitions when the iSCSI connection
is not waiting for, or undergoing, a cleanup by way of an explicit or
implicit logout. The second set, "connection cleanup state diagram",
describes the connection state transitions while performing the iSCSI
connection cleanup. While the first set has two diagrams -- one each
for initiator and target -- the second set has a single diagram
applicable to both initiators and targets.

The "session state diagram" describes the state transitions an iSCSI
session would go through during its lifetime, and it depends on the
states of possibly multiple iSCSI connections that participate in the
session.

States and transitions are described in text, tables, and diagrams.
The diagrams are used for illustration. The text and the tables are
the governing specification.

8.1. Standard Connection State Diagrams

8.1.1. State Descriptions for Initiators and Targets

State descriptions for the standard connection state diagram are as
follows:

S1:

FREE

initiator: State on instantiation, or after successful
connection closure.

target: State on instantiation, or after successful
connection closure.

S2:

XPT_WAIT

initiator: Waiting for a response to its transport
connection establishment request.

target: Illegal.

S3:

XPT_UP

initiator: Illegal.

target: Waiting for the login process to commence.

S4:

IN_LOGIN

initiator: Waiting for the login process to conclude,
possibly involving several PDU exchanges.

target: Waiting for the login process to conclude,
possibly involving several PDU exchanges.

S5:

LOGGED_IN

initiator: In the Full Feature Phase, waiting for all
internal, iSCSI, and transport events.

target: In the Full Feature Phase, waiting for all internal,
iSCSI, and transport events.

initiator: Waiting for an internal event signaling
readiness to proceed with Logout.

target: Waiting for the Logout process to start after
having requested a Logout via an Async Message.

S8:

CLEANUP_WAIT

initiator: Waiting for the context and/or resources to
initiate the cleanup processing for this CSM.

target: Waiting for the cleanup process to start for this CSM.

8.1.2. State Transition Descriptions for Initiators and Targets

T1:

- initiator: Transport connect request was made (e.g., TCP SYN
sent).

target: Illegal.

T2:

initiator: Transport connection request timed out, a
transport reset was received, or an internal event of
receiving a Logout Response (success) on another connection
for a "close the session" Logout Request was received.

target: Illegal.

T3:

initiator: Illegal.

target: Received a valid transport connection request that
establishes the transport connection.

initiator: The final iSCSI Login Response with a Status-Class
of zero was received.

target: The final iSCSI Login Request to conclude the
Login Phase was received, thus prompting the target to send
the final iSCSI Login Response with a Status-Class of zero.

T6:

initiator: Illegal.

target: Timed out waiting for an iSCSI Login, transport
disconnect indication was received, transport reset was
received, or an internal event indicating a transport
timeout was received. In all these cases, the connection is
to be closed.

T7:

initiator: One of the following events caused the transition:

a) The final iSCSI Login Response was received with a

non-zero Status-Class.

b) Login timed out.

c) A transport disconnect indication was received.

d) A transport reset was received.

e) An internal event indicating a transport timeout was

received.

f) An internal event of receiving a Logout Response

(success) on another connection for a "close the

session" Logout Request was received.

In all these cases, the transport connection is closed.

target: One of the following events caused the transition:

a) The final iSCSI Login Request to conclude the Login

Phase was received, prompting the target to send the

final iSCSI Login Response with a non-zero Status-Class.

b) Login timed out.

c) A transport disconnect indication was received.

d) A transport reset was received.

e) An internal event indicating a transport timeout was

received.

f) On another connection, a "close the session" Logout Request

was received.

In all these cases, the connection is to be closed.

T8:

initiator: An internal event of receiving a Logout
Response (success) on another connection for a "close the
session" Logout Request was received, thus closing this
connection and requiring no further cleanup.

target: An internal event of sending a Logout Response
(success) on another connection for a "close the session"
Logout Request was received, or an internal event of a
successful connection/session reinstatement was received,
thus prompting the target to close this connection cleanly.

T9, T10:

initiator: An internal event that indicates the readiness
to start the Logout process was received, thus prompting an
iSCSI Logout to be sent by the initiator.

target: An iSCSI Logout Request was received.

T11, T12:

initiator: An Async PDU with AsyncEvent "Request Logout"
was received.

target: An internal event that requires the decommissioning
of the connection was received, thus causing an Async PDU with
an AsyncEvent "Request Logout" to be sent.

T13:

initiator: An iSCSI Logout Response (success) was received,
or an internal event of receiving a Logout Response (success)
on another connection for a "close the session" Logout Request
was received.

target: An internal event was received that indicates
successful processing of the Logout, which prompts an iSCSI
Logout Response (success) to be sent; an internal event of
sending a Logout Response (success) on another connection
for a "close the session" Logout Request was received; or

an internal event of a successful connection/session

reinstatement was received. In all these cases, the

transport connection is closed.

T14:

initiator: An Async PDU with AsyncEvent "Request Logout"
was received again.

target: Illegal.

T15, T16:

initiator: One or more of the following events caused this
transition:

Logout processing was received, which prompts a

Logout Response (failure, i.e., a non-zero status)

to be sent.

b) Any of the events specified for T15 and T16 occurred.

T18:

initiator: An internal event of receiving a Logout
Response (success) on another connection for a "close the
session" Logout Request was received.

target: An internal event of sending a Logout Response
(success) on another connection for a "close the session"
Logout Request was received, or an internal event of a
successful connection/session reinstatement was received.
In both these cases, the connection is closed.

The CLEANUP_WAIT state (S8) implies that there are possible iSCSI
tasks that have not reached conclusion and are still considered
busy.

8.1.3. Standard Connection State Diagram for an Initiator

Symbolic names for states:

S1:

FREE

S2:

XPT_WAIT

S4:

IN_LOGIN

S5:

LOGGED_IN

S6:

IN_LOGOUT

S7:

LOGOUT_REQUESTED

S8:

CLEANUP_WAIT

States S5, S6, and S7 constitute the Full Feature Phase operation of
the connection.

The following state transition table represents the above diagram.
Each row represents the starting state for a given transition, which,
after taking a transition marked in a table cell, would end in the
state represented by the column of the cell. For example, from
state S1, the connection takes the T1 transition to arrive at
state S2. The fields marked "-" correspond to undefined transitions.

8.2. Connection Cleanup State Diagram for Initiators and Targets

Symbolic names for states:

R1: CLEANUP_WAIT (same as S8)

R2:

IN_CLEANUP
R3: FREE (same as S1)

Whenever a connection state machine in cleanup (let's call it CSM-C)
enters the CLEANUP_WAIT state (S8), it must go through the state
transitions described in the connection cleanup state diagram, using
either a) a separate Full Feature Phase connection (let's call it
CSM-E, for explicit) in the LOGGED_IN state in the same session or
b) a new transport connection (let's call it CSM-I, for implicit) in
the FREE state that is to be added to the same session. In the CSM-E
case, an explicit logout for the CID that corresponds to CSM-C (as
either a connection or session logout) needs to be performed to
complete the cleanup. In the CSM-I case, an implicit logout for the
CID that corresponds to CSM-C needs to be performed by way of
connection reinstatement (Section 6.3.4) for that CID. In either
case, the protocol exchanges on CSM-E or CSM-I determine the state
transitions for CSM-C. Therefore, this cleanup state diagram is only
applicable to the instance of the connection in cleanup (i.e.,
CSM-C). In the case of an implicit logout, for example, CSM-C
reaches FREE (R3) at the time CSM-I reaches LOGGED_IN. In the case
of an explicit logout, CSM-C reaches FREE (R3) when CSM-E receives a
successful Logout Response while continuing to be in the LOGGED_IN
state.

An initiator must initiate an explicit or implicit connection logout
for a connection in the CLEANUP_WAIT state, if the initiator intends
to continue using the associated iSCSI session.

The following state diagram applies to both initiators and targets.
(M1, M2, M3, and M4 are defined in Section 8.2.2.)

initiator: either CSM-E moved out of LOGGED_IN, or Logout
timed out and/or aborted, or Logout Response (failure) was
received.

target: either CSM-E moved out of LOGGED_IN, Logout timed out
and/or aborted, or an internal event that indicates that a
failed Logout processing was received. A Logout Response
(failure) was sent in the last case.

M4:

Successful implicit/explicit logout was performed.

In CSM-I usage:

* initiator: CSM-I reached state LOGGED_IN, or an internal
event of receiving a Logout Response (success) on another
connection for a "close the session" Logout Request was
received.

target: CSM-I reached state LOGGED_IN, or an internal event
of sending a Logout Response (success) on a different
connection for a "close the session" Logout Request was
received.

In CSM-E usage:

initiator: CSM-E stayed in LOGGED_IN and received a Logout
Response (success), or an internal event of receiving a
Logout Response (success) on another connection for a "close
the session" Logout Request was received.

target: CSM-E stayed in LOGGED_IN and an internal event
indicating a successful Logout processing was received, or an
internal event of sending a Logout Response (success) on a
different connection for a "close the session" Logout Request
was received.

9. Security Considerations

Historically, native storage systems have not had to consider
security, because their environments offered minimal security risks.
That is, these environments consisted of storage devices either
directly attached to hosts or connected via a Storage Area Network
(SAN) distinctly separate from the communications network. The use
of storage protocols, such as SCSI, over IP networks requires that
security concerns be addressed. iSCSI implementations must provide
means of protection against active attacks (e.g., pretending to be
another identity; message insertion, deletion, modification, and
replaying) and passive attacks (e.g., eavesdropping, gaining
advantage by analyzing the data sent over the line).

Although technically possible, iSCSI SHOULD NOT be configured without
security, specifically in-band authentication; see Section 9.2.
iSCSI configured without security should be confined to closed
environments that have very limited and well-controlled security
risks. [RFC3723] specifies the mechanisms that must be used in order
to mitigate risks fully described in that document.

The following section describes the security mechanisms provided by
an iSCSI implementation.

9.1. iSCSI Security Mechanisms

The entities involved in iSCSI security are the initiator, target,
and the IP communication endpoints. iSCSI scenarios in which
multiple initiators or targets share a single communication endpoint
are expected. To accommodate such scenarios, iSCSI supports two
separate security mechanisms: in-band authentication between the
initiator and the target at the iSCSI connection level (carried out
by exchange of iSCSI Login PDUs), and packet protection (integrity,
authentication, and confidentiality) by IPsec at the IP level. The
two security mechanisms complement each other. The in-band
authentication provides end-to-end trust (at login time) between the
iSCSI initiator and the target, while IPsec provides a secure channel
between the IP communication endpoints. iSCSI can be used to access
sensitive information for which significant security protection is
appropriate. As further specified in the rest of this security
considerations section, both iSCSI security mechanisms are mandatory
to implement (MUST). The use of in-band authentication is strongly
recommended (SHOULD). In contrast, the use of IPsec is optional
(MAY), as the security risks that it addresses may only be present
over a subset of the networks used by an iSCSI connection or a
session; a specific example is that when an iSCSI session spans data
centers, IPsec VPN gateways at the data center boundaries to protect
the WAN connectivity between data centers may be appropriate in
combination with in-band iSCSI authentication.

Further details on typical iSCSI scenarios and the relationship
between the initiators, targets, and the communication endpoints can
be found in [RFC3723].

9.2. In-Band Initiator-Target Authentication

During login, the target MAY authenticate the initiator and the
initiator MAY authenticate the target. The authentication is
performed on every new iSCSI connection by an exchange of iSCSI Login
PDUs using a negotiated authentication method.

The authentication method cannot assume an underlying IPsec
protection, because IPsec is optional to use. An attacker should
gain as little advantage as possible by inspecting the authentication
phase PDUs. Therefore, a method using cleartext (or equivalent)
passwords MUST NOT be used; on the other hand, identity protection is
not strictly required.

The authentication mechanism protects against an unauthorized login
to storage resources by using a false identity (spoofing). Once the
authentication phase is completed, if the underlying IPsec is not
used, all PDUs are sent and received in the clear. The
authentication mechanism alone (without underlying IPsec) should only
be used when there is no risk of eavesdropping or of message
insertion, deletion, modification, and replaying.

Section 12 defines several authentication methods and the exact steps
that must be followed in each of them, including the iSCSI-text-keys
and their allowed values in each step. Whenever an iSCSI initiator
gets a response whose keys, or their values, are not according to the
step definition, it MUST abort the connection.

Whenever an iSCSI target gets a request or response whose keys, or
their values, are not according to the step definition, it MUST
answer with a Login reject with the "Initiator Error" or "Missing
Parameter" status. These statuses are not intended for
cryptographically incorrect values such as the CHAP response, for
which the "Authentication Failure" status MUST be specified. The
importance of this rule can be illustrated in CHAP with target
authentication (see Section 12.1.3), where the initiator would have
been able to conduct a reflection attack by omitting its response key
(CHAP_R), using the same CHAP challenge as the target and reflecting
the target's response back to the target. In CHAP, this is prevented
because the target must answer the missing CHAP_R key with a
Login reject with the "Missing Parameter" status.

For some of the authentication methods, a key specifies the identity
of the iSCSI initiator or target for authentication purposes. The
value associated with that key MAY be different from the iSCSI name
and SHOULD be configurable (CHAP_N: see Section 12.1.3; SRP_U: see
Section 12.1.2). For this reason, iSCSI implementations SHOULD
manage authentication in a way that impersonation across iSCSI names
via these authentication identities is not possible. Specifically,
implementations SHOULD allow configuration of an authentication
identity for a Name if different, and authentication credentials for
that identity. During the login time, implementations SHOULD verify
the Name-to-identity relationship in addition to authenticating the
identity through the negotiated authentication method.

When an iSCSI session has multiple TCP connections, either
concurrently or sequentially, the authentication method and
identities should not vary among the connections. Therefore, all
connections in an iSCSI session SHOULD use the same authentication
method, iSCSI name, and authentication identity (for authentication
methods that use an authentication identity). Implementations SHOULD
check this and cause an authentication failure on a new connection
that uses a different authentication method, iSCSI name, or
authentication identity from those already used in the session. In
addition, implementations SHOULD NOT support both authenticated and
unauthenticated TCP connections in the same iSCSI session, added
either concurrently or sequentially to the session.

9.2.1. CHAP Considerations

When CHAP is performed over a non-encrypted channel, it is vulnerable
to an off-line dictionary attack. Implementations MUST support the
use of up to 128-bit random CHAP secrets, including the means to
generate such secrets and to accept them from an external generation
source. Implementations MUST NOT provide secret generation (or
expansion) means other than random generation.

An administrative entity of an environment in which CHAP is used with
a secret that has less than 96 random bits MUST enforce IPsec
encryption (according to the implementation requirements in
Section 9.3.2) to protect the connection. Moreover, in this case,
IKE authentication with group pre-shared cryptographic keys SHOULD
NOT be used unless it is not essential to protect group members
against off-line dictionary attacks by other members.

CHAP secrets MUST be an integral number of bytes (octets). A
compliant implementation SHOULD NOT continue with the login step in
which it should send a CHAP response (CHAP_R; see Section 12.1.3)
unless it can verify that the CHAP secret is at least 96 bits or that
IPsec encryption is being used to protect the connection.

Any CHAP secret used for initiator authentication MUST NOT be
configured for authentication of any target, and any CHAP secret used
for target authentication MUST NOT be configured for authentication
of any initiator. If the CHAP response received by one end of an
iSCSI connection is the same as the CHAP response that the receiving
endpoint would have generated for the same CHAP challenge, the
response MUST be treated as an authentication failure and cause the
connection to close (this ensures that the same CHAP secret is not
used for authentication in both directions). Also, if an iSCSI
implementation can function as both initiator and target, different
CHAP secrets and identities MUST be configured for these two roles.
The following is an example of the attacks prevented by the above
requirements:

a) "Rogue" wants to impersonate "Storage" to Alice and knows that

a single secret is used for both directions of Storage-Alice
authentication.

b) Rogue convinces Alice to open two connections to itself and

identifies itself as Storage on both connections.

c) Rogue issues a CHAP challenge on Connection 1, waits for Alice

to respond, and then reflects Alice's challenge as the initial
challenge to Alice on Connection 2.

d) If Alice doesn't check for the reflection across connections,

Alice's response on Connection 2 enables Rogue to impersonate
Storage on Connection 1, even though Rogue does not know the
Alice-Storage CHAP secret.

Originators MUST NOT reuse the CHAP challenge sent by the responder
for the other direction of a bidirectional authentication.
Responders MUST check for this condition and close the iSCSI TCP
connection if it occurs.

The same CHAP secret SHOULD NOT be configured for authentication of
multiple initiators or multiple targets, as this enables any of them
to impersonate any other one of them, and compromising one of them
enables the attacker to impersonate any of them. It is recommended
that iSCSI implementations check for the use of identical CHAP
secrets by different peers when this check is feasible and take
appropriate measures to warn users and/or administrators when this is
detected.

When an iSCSI initiator or target authenticates itself to
counterparts in multiple administrative domains, it SHOULD use a
different CHAP secret for each administrative domain to avoid
propagating security compromises across domains.

Within a single administrative domain:

A single CHAP secret MAY be used for authentication of an
initiator to multiple targets.

A single CHAP secret MAY be used for an authentication of a
target to multiple initiators when the initiators use an
external server (e.g., RADIUS [RFC2865]) to verify the target's
CHAP responses and do not know the target's CHAP secret.

If an external response verification server (e.g., RADIUS) is not
used, employing a single CHAP secret for authentication of a target
to multiple initiators requires that all such initiators know that
target's secret. Any of these initiators can impersonate the target
to any other such initiator, and compromise of such an initiator
enables an attacker to impersonate the target to all such initiators.
Targets SHOULD use separate CHAP secrets for authentication to each
initiator when such risks are of concern; in this situation, it may
be useful to configure a separate logical iSCSI target with its own
iSCSI Node Name for each initiator or group of initiators among which
such separation is desired.

The above requirements strengthen the security properties of CHAP
authentication for iSCSI by comparison to the basic CHAP
authentication mechanism [RFC1994]. It is very important to adhere
to these requirements, especially the requirements for strong (large
randomly generated) CHAP secrets, as iSCSI implementations and
deployments that fail to use strong CHAP secrets are likely to be
highly vulnerable to off-line dictionary attacks on CHAP secrets.

Replacement of CHAP with a better authentication mechanism is
anticipated in a future version of iSCSI. The FC-SP-2 standard
[FC-SP-2] has specified the Extensible Authentication Protocol -
Generalized Pre-Shared Key (EAP-GPSK) authentication mechanism
[RFC5433] as an alternative to (and possible future replacement for)
Fibre Channel's similar usage of strengthened CHAP. Another possible
replacement for CHAP is a secure password mechanism, e.g., an updated
version of iSCSI's current SRP authentication mechanism.

9.2.2. SRP Considerations

The strength of the SRP authentication method (specified in
[RFC2945]) is dependent on the characteristics of the group being
used (i.e., the prime modulus N and generator g). As described in
[RFC2945], N is required to be a Sophie Germain prime (of the form
N = 2q + 1, where q is also prime) and the generator g is a primitive
root of GF(N). In iSCSI authentication, the prime modulus N MUST be
at least 768 bits.

9.2.3. Kerberos Considerations

iSCSI uses raw Kerberos V5 [RFC4120] for authenticating a client
(iSCSI initiator) principal to a service (iSCSI target) principal.
Note that iSCSI does not use the Generic Security Service Application
Program Interface (GSS-API) [RFC2743] or the Kerberos V5 GSS-API
security mechanism [RFC4121]. This means that iSCSI implementations
supporting the KRB5 AuthMethod (Section 12.1) are directly involved
in the Kerberos protocol. When Kerberos V5 is used for
authentication, the following actions MUST be performed as specified
in [RFC4120]:

The target MUST validate KRB_AP_REQ to ensure that the initiator
can be trusted.

When mutual authentication is selected, the initiator MUST
validate KRB_AP_REP to determine the outcome of mutual
authentication.

As Kerberos V5 is capable of providing mutual authentication,
implementations SHOULD support mutual authentication by default for
login authentication.

Note, however, that Kerberos authentication only assures that the
server (iSCSI target) can be trusted by the Kerberos client
(initiator) and vice versa; an initiator should employ appropriately
secured service discovery techniques (e.g., iSNS; see Section 4.2.7)
to ensure that it is talking to the intended target principal.

iSCSI does not use Kerberos v5 for either integrity or
confidentiality protection of the iSCSI protocol. iSCSI uses IPsec
for those purposes as specified in Section 9.3.

9.3. IPsec

iSCSI uses the IPsec mechanism for packet protection (cryptographic
integrity, authentication, and confidentiality) at the IP level
between the iSCSI communicating endpoints. The following sections
describe the IPsec protocols that must be implemented for data
authentication and integrity; confidentiality; and cryptographic key
management.

An iSCSI initiator or target may provide the required IPsec support
fully integrated or in conjunction with an IPsec front-end device.
In the latter case, the compliance requirements with regard to IPsec
support apply to the "combined device". Only the "combined device"
is to be considered an iSCSI device.

Detailed considerations and recommendations for using IPsec for iSCSI
are provided in [RFC3723] as updated by [RFC7146]. The IPsec
requirements are reproduced here for convenience and are intended to
match those in [RFC7146]; in the event of a discrepancy, the
requirements in [RFC7146] apply.

9.3.1. Data Authentication and Integrity

Data authentication and integrity are provided by a cryptographic
keyed Message Authentication Code in every sent packet. This code
protects against message insertion, deletion, and modification.
Protection against message replay is realized by using a sequence
counter.

An iSCSI-compliant initiator or target MUST provide data
authentication and integrity by implementing IPsec v2 [RFC2401] with
ESPv2 [RFC2406] in tunnel mode, SHOULD provide data authentication
and integrity by implementing IPsec v3 [RFC4301] with ESPv3 [RFC4303]
in tunnel mode, and MAY provide data authentication and integrity by
implementing either IPsec v2 or v3 with the appropriate version of
ESP in transport mode. The IPsec implementation MUST fulfill the
following iSCSI-specific requirements:

HMAC-SHA1 MUST be implemented in the specific form of
HMAC-SHA-1-96 [RFC2404].

AES CBC MAC with XCBC extensions using 128-bit keys SHOULD be
implemented [RFC3566].

At the high speeds at which iSCSI is expected to operate, a single
IPsec SA could rapidly exhaust the ESP 32-bit sequence number space,
requiring frequent rekeying of the SA, as rollover of the ESP
sequence number within a single SA is prohibited for both ESPv2
[RFC2406] and ESPv3 [RFC4303]. In order to provide the means to
avoid this potentially undesirable frequent rekeying, implementations
that are capable of operating at speeds of 1 gigabit/second or higher
MUST implement extended (64-bit) sequence numbers for ESPv2 (and
ESPv3, if supported) and SHOULD use extended sequence numbers for all
iSCSI traffic. Extended sequence number negotiation as part of
security association establishment is specified in [RFC4304] for
IKEv1 and [RFC5996] for IKEv2.

9.3.2. Confidentiality

Confidentiality is provided by encrypting the data in every packet.
When confidentiality is used, it MUST be accompanied by data
authentication and integrity to provide comprehensive protection
against eavesdropping and against message insertion, deletion,
modification, and replaying.

An iSCSI-compliant initiator or target MUST provide confidentiality
by implementing IPsec v2 [RFC2401] with ESPv2 [RFC2406] in tunnel
mode, SHOULD provide confidentiality by implementing IPsec v3
[RFC4301] with ESPv3 [RFC4303] in tunnel mode, and MAY provide
confidentiality by implementing either IPsec v2 or v3 with the
appropriate version of ESP in transport mode, with the following
iSCSI-specific requirements that apply to IPsec v2 and IPsec v3:

A compliant iSCSI implementation MUST meet the cryptographic key
management requirements of the IPsec protocol suite. Authentication,
security association negotiation, and cryptographic key management
MUST be provided by implementing IKE [RFC2409] using the IPsec DOI
[RFC2407] and SHOULD be provided by implementing IKEv2 [RFC5996],
with the following iSCSI-specific requirements:

a) Peer authentication using a pre-shared cryptographic key MUST

be supported. Certificate-based peer authentication using
digital signatures MAY be supported. For IKEv1 ([RFC2409]),
peer authentication using the public key encryption methods
outlined in Sections 5.2 and 5.3 of [RFC2409] SHOULD NOT be
used.

b) When digital signatures are used to achieve authentication, an

IKE negotiator SHOULD use IKE Certificate Request Payload(s) to
specify the certificate authority. IKE negotiators SHOULD
check certificate validity via the pertinent Certificate
Revocation List (CRL) or via the use of the Online Certificate
Status Protocol (OCSP) [RFC6960] before accepting a PKI
certificate for use in IKE authentication procedures. OCSP
support within the IKEv2 protocol is specified in [RFC4806].
These checks may not be needed in environments where a small
number of certificates are statically configured as trust
anchors.

c) Conformant iSCSI implementations of IKEv1 MUST support Main

Mode and SHOULD support Aggressive Mode. Main Mode with a
pre-shared key authentication method SHOULD NOT be used when
either the initiator or the target uses dynamically assigned
addresses. While in many cases pre-shared keys offer good
security, situations in which dynamically assigned addresses
are used force the use of a group pre-shared key, which creates
vulnerability to a man-in-the-middle attack.

d) In the IKEv1 Phase 2 Quick Mode, in exchanges for creating the

Phase 2 SA, the Identification Payload MUST be present.

e) The following identification type requirements apply to IKEv1:

ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack supports
IPv6), and ID_FQDN Identification Types MUST be supported;
ID_USER_FQDN SHOULD be supported. The IP Subnet, IP Address
Range, ID_DER_ASN1_DN, and ID_DER_ASN1_GN Identification Types
SHOULD NOT be used. The ID_KEY_ID Identification Type MUST NOT
be used.

f) If IKEv2 is supported, the following identification

requirements apply: ID_IPV4_ADDR, ID_IPV6_ADDR (if the
protocol stack supports IPv6), and ID_FQDN Identification Types
MUST be supported; ID_RFC822_ADDR SHOULD be supported. The
ID_DER_ASN1_DN and ID_DER_ASN1_GN Identification Types SHOULD
NOT be used. The ID_KEY_ID Identification Type MUST NOT be
used.

The reasons for the "MUST NOT" and "SHOULD NOT" for identification
type requirements in preceding bullets e) and f) are:

The DN and GN types are X.500 identities; it is usually better
to use an identity from subjectAltName in a PKI certificate.

ID_KEY_ID is not interoperable as specified.

Manual cryptographic keying MUST NOT be used, because it does not
provide the necessary rekeying support.

When Diffie-Hellman (DH) groups are used, a DH group of at least
2048 bits SHOULD be offered as a part of all proposals to create
IPsec security associations to protect iSCSI traffic, with both IKEv1
and IKEv2.

When IPsec is used, the receipt of an IKEv1 Phase 2 delete message or
an IKEv2 INFORMATIONAL exchange that deletes the SA SHOULD NOT be
interpreted as a reason for tearing down the iSCSI TCP connection.
If additional traffic is sent on it, a new IKE SA will be created to
protect it.

The method used by the initiator to determine whether the target
should be connected using IPsec is regarded as an issue of IPsec
policy administration and thus not defined in the iSCSI standard.

The method used by an initiator that supports both IPsec v2 and v3 to
determine which versions of IPsec are supported by the target is also
regarded as an issue of IPsec policy administration and thus not
defined in the iSCSI standard. If both IPsec v2 and v3 are supported
by both the initiator and target, the use of IPsec v3 is recommended.

If an iSCSI target is discovered via a SendTargets request in a
Discovery session not using IPsec, the initiator should assume that
it does not need IPsec to establish a session to that target. If an
iSCSI target is discovered using a Discovery session that does use
IPsec, the initiator SHOULD use IPsec when establishing a session to
that target.

9.4. Security Considerations for the X#NodeArchitecture Key

The security considerations in this section are specific to the
X#NodeArchitecture discussed in Section 13.26.

This extension key transmits specific implementation details about
the node that sends it; such details may be considered sensitive in
some environments. For example, if a certain software or firmware
version is known to contain security weaknesses, announcing the
presence of that version via this key may not be desirable. The
countermeasures for this security concern are:

a) sending less detailed information in the key values,

b) not sending the extension key, or

c) using IPsec ([RFC4303]) to provide confidentiality for the
iSCSI connection on which the key is sent.

To support the first and second countermeasures, all implementations
of this extension key MUST provide an administrative mechanism to
disable sending the key. In addition, all implementations SHOULD
provide an administrative mechanism to configure a verbosity level of
the key value, thereby controlling the amount of information sent.

For example, a lower verbosity level might enable transmission of
node architecture component names only, but no version numbers. The
choice of which countermeasure is most appropriate depends on the
environment. However, sending less detailed information in the key
values may be an acceptable countermeasure in many environments,
since it provides a compromise between sending too much information
and the other more complete countermeasures of not sending the key at
all or using IPsec.

In addition to security considerations involving transmission of the
key contents, any logging method(s) used for the key values MUST keep
the information secure from intruders. For all implementations, the
requirements to address this security concern are as follows:

a) Display of the log MUST only be possible with administrative

rights to the node.

b) Options to disable logging to disk and to keep logs for a fixed

duration SHOULD be provided.

Finally, it is important to note that different nodes may have
different levels of risk, and these differences may affect the
implementation. The components of risk include assets, threats, and
vulnerabilities. Consider the following example iSCSI nodes, which
demonstrate differences in assets and vulnerabilities of the nodes,
and, as a result, differences in implementation:

a) One iSCSI target based on a special-purpose operating system:

Since the iSCSI target controls access to the data storage
containing company assets, the asset level is seen as very
high. Also, because of the special-purpose operating system,
in which vulnerabilities are less well known, the vulnerability
level is viewed as low.

b) Multiple iSCSI initiators in a blade farm, each running a

general-purpose operating system: The asset level of each node
is viewed as low, since blades are replaceable and low cost.
However, the vulnerability level is viewed as high, since there
may be many well-known vulnerabilities to that general-purpose
operating system. For this target, an appropriate
implementation might be the logging of received key values but
no transmission of the key. For this initiator, an appropriate
implementation might be transmission of the key but no logging
of received key values.

9.5. SCSI Access Control Considerations

iSCSI is a SCSI transport protocol and as such does not apply any
access controls on SCSI-level operations such as SCSI task management
functions (e.g., LU reset; see Section 11.5.1). SCSI-level access
controls (e.g., ACCESS CONTROL OUT; see [SPC3]) have to be
appropriately deployed in practice to address SCSI-level security
considerations, in addition to security via iSCSI connection and
packet protection mechanisms that were already discussed in preceding
sections.

10. Notes to Implementers

This section notes some of the performance and reliability
considerations of the iSCSI protocol. This protocol was designed to
allow efficient silicon and software implementations. The iSCSI task
tag mechanism was designed to enable Direct Data Placement (DDP -- a
DMA form) at the iSCSI level or lower.

The guiding assumption made throughout the design of this protocol is
that targets are resource constrained relative to initiators.

Implementers are also advised to consider the implementation
consequences of the iSCSI-to-SCSI mapping model as outlined in
Section 4.4.3.

10.1. Multiple Network Adapters

The iSCSI protocol allows multiple connections, not all of which need
to go over the same network adapter. If multiple network connections
are to be utilized with hardware support, the iSCSI protocol command-
data-status allegiance to one TCP connection ensures that there is no
need to replicate information across network adapters or otherwise
require them to cooperate.

However, some task management commands may require some loose form of
cooperation or replication at least on the target.

10.1.1. Conservative Reuse of ISIDs

Historically, the SCSI model (and implementations and applications
based on that model) has assumed that SCSI ports are static, physical
entities. Recent extensions to the SCSI model have taken advantage
of persistent worldwide unique names for these ports. In iSCSI,
however, the SCSI initiator ports are the endpoints of dynamically
created sessions, so the presumptions of "static and physical" do not
apply. In any case, the "model" sections (particularly,

Section 4.4.1) provide for persistent, reusable names for the
iSCSI-type SCSI initiator ports even though there does not need to be
any physical entity bound to these names.

To both minimize the disruption of legacy applications and better
facilitate the SCSI features that rely on persistent names for SCSI
ports, iSCSI implementations SHOULD attempt to provide a stable
presentation of SCSI initiator ports (both to the upper OS layers and
the targets to which they connect). This can be achieved in an
initiator implementation by conservatively reusing ISIDs. In other
words, the same ISID should be used in the login process to multiple
target portal groups (of the same iSCSI target or different iSCSI
targets). The ISID RULE (Section 4.4.3) only prohibits reuse to the
same target portal group. It does not "preclude" reuse to other
target portal groups. The principle of conservative reuse
"encourages" reuse to other target portal groups. When a SCSI target
device sees the same (InitiatorName, ISID) pair in different sessions
to different target portal groups, it can identify the underlying
SCSI initiator port on each session as the same SCSI port. In
effect, it can recognize multiple paths from the same source.

10.1.2. iSCSI Name, ISID, and TPGT Use

The designers of the iSCSI protocol are aware that legacy SCSI
transports rely on initiator identity to assign access to storage
resources. Although newer techniques that simplify access control
are available, support for configuration and authentication schemes
that are based on initiator identity is deemed important in order to
support legacy systems and administration software. iSCSI thus
supports the notion that it should be possible to assign access to
storage resources based on "initiator device" identity.

When there are multiple hardware or software components coordinated
as a single iSCSI node, there must be some (logical) entity that
represents the iSCSI node that makes the iSCSI Node Name available to
all components involved in session creation and login. Similarly,
this entity that represents the iSCSI node must be able to coordinate
session identifier resources (the ISID for initiators) to enforce
both the ISID RULE and the TSIH RULE (see Section 4.4.3).

For targets, because of the closed environment, implementation of
this entity should be straightforward. However, vendors of iSCSI
hardware (e.g., NICs or HBAs) intended for targets SHOULD provide
mechanisms for configuration of the iSCSI Node Name across the portal
groups instantiated by multiple instances of these components within
a target.

However, complex targets making use of multiple Target Portal Group
Tags may reconfigure them to achieve various quality goals. The
initiators have two mechanisms at their disposal to discover and/or
check reconfiguring targets -- the Discovery session type and a key
returned by the target during login to confirm the TPGT. An
initiator should attempt to "rediscover" the target configuration
whenever a session is terminated unexpectedly.

For initiators, in the long term, it is expected that operating
system vendors will take on the role of this entity and provide
standard APIs that can inform components of their iSCSI Node Name and
can configure and/or coordinate ISID allocation, use, and reuse.

Recognizing that such initiator APIs are not available today, other
implementations of the role of this entity are possible. For
example, a human may instantiate the (common) node name as part of
the installation process of each iSCSI component involved in session
creation and login. This may be done by pointing the component to
either a vendor-specific location for this datum or a system-wide
location. The structure of the ISID namespace (see Section 11.12.5
and [RFC3721]) facilitates implementation of the ISID coordination by
allowing each component vendor to independently (of other vendor's
components) coordinate allocation, use, and reuse of its own
partition of the ISID namespace in a vendor-specific manner.
Partitioning of the ISID namespace within initiator portal groups
managed by that vendor allows each such initiator portal group to act
independently of all other portal groups when selecting an ISID for a
login; this facilitates enforcement of the ISID RULE (see
Section 4.4.3) at the initiator.

A vendor of iSCSI hardware (e.g., NICs or HBAs) intended for use in
initiators MUST implement a mechanism for configuring the iSCSI Node
Name. Vendors and administrators must ensure that iSCSI Node Names
are worldwide unique. It is therefore important that when one
chooses to reuse the iSCSI Node Name of a disabled unit one does not
reassign that name to the original unit unless its worldwide
uniqueness can be ascertained again.

In addition, a vendor of iSCSI hardware must implement a mechanism to
configure and/or coordinate ISIDs for all sessions managed by
multiple instances of that hardware within a given iSCSI node. Such
configuration might be either permanently preassigned at the factory
(in a necessarily globally unique way), statically assigned (e.g.,
partitioned across all the NICs at initialization in a locally unique
way), or dynamically assigned (e.g., on-line allocator, also in a
locally unique way). In the latter two cases, the configuration may
be via public APIs (perhaps driven by an independent vendor's
software, such as the OS vendor) or private APIs driven by the
vendor's own software.

The process of name assignment and coordination has to be as
encompassing and automated as possible, as years of legacy usage have
shown that it is highly error-prone. It should be mentioned that
today SCSI has alternative schemes of access control that can be used
by all transports, and their security is not dependent on strict
naming coordination.

10.2. Autosense and Auto Contingent Allegiance (ACA)

"Autosense" refers to the automatic return of sense data to the
initiator in cases where a command did not complete successfully.
iSCSI initiators and targets MUST support and use Autosense.

ACA helps preserve ordered command execution in the presence of
errors. As there can be many commands in-flight between an initiator
and a target, SCSI initiator functionality in some operating systems
depends on ACA to enforce ordered command execution during error
recovery, and hence iSCSI initiator implementations for those
operating systems need to support ACA. In order to support error
recovery for these operating systems and iSCSI initiators, iSCSI
targets SHOULD support ACA.

10.3. iSCSI Timeouts

iSCSI recovery actions are often dependent on iSCSI timeouts being
recognized and acted upon before SCSI timeouts. Determining the
right timeouts to use for various iSCSI actions (command
acknowledgments expected, status acknowledgments, etc.) is very much
dependent on infrastructure (e.g., hardware, links, TCP/IP stack,
iSCSI driver). As a guide, the implementer may use an average
NOP-Out/NOP-In turnaround delay multiplied by a "safety factor"
(e.g., 4) as a good estimate for the basic delay of the iSCSI stack
for a given connection. The safety factor should account for network
load variability. For connection teardown, the implementer may want
to also consider TCP common practice for the given infrastructure.

Text negotiations MAY also be subject to either time limits or limits
in the number of exchanges. Those limits SHOULD be generous enough
to avoid affecting interoperability (e.g., allowing each key to be
negotiated on a separate exchange).

The relationship between iSCSI timeouts and SCSI timeouts should also
be considered. SCSI timeouts should be longer than iSCSI timeouts
plus the time required for iSCSI recovery whenever iSCSI recovery is
planned. Alternatively, an implementer may choose to interlock iSCSI
timeouts and recovery with SCSI timeouts so that SCSI recovery will
become active only where iSCSI is not planned to, or failed to,
recover.

The implementer may also want to consider the interaction between
various iSCSI exception events -- such as a digest failure -- and
subsequent timeouts. When iSCSI error recovery is active, a digest
failure is likely to result in discovering a missing command or data
PDU. In these cases, an implementer may want to lower the timeout
values to enable faster initiation for recovery procedures.

10.4. Command Retry and Cleaning Old Command Instances

To avoid having old, retried command instances appear in a valid
command window after a command sequence number wraparound, the
protocol requires (see Section 4.2.2.1) that on every connection on
which a retry has been issued a non-immediate command be issued and
acknowledged within an interval of 2**31 - 1 commands from the CmdSN
of the retried command. This requirement can be fulfilled by an
implementation in several ways.

The simplest technique to use is to send a (non-retry) non-immediate
SCSI command (or a NOP if no SCSI command is available for a while)
after every command retry on the connection on which the retry was
attempted. Because errors are deemed rare events, this technique is
probably the most effective, as it does not involve additional checks
at the initiator when issuing commands.

10.5. Sync and Steering Layer, and Performance

While a Sync and Steering layer is optional, an initiator/target that
does not have it working against a target/initiator that demands sync
and steering may experience performance degradation caused by packet
reordering and loss. Providing a sync and steering mechanism is
recommended for all high-speed implementations.

Operations

Sequential access devices operate on the principle that the position
of the device is based on the last command processed. As such,
command processing order, and knowledge of whether or not the
previous command was processed, are of the utmost importance to
maintain data integrity. For example, inadvertent retries of SCSI
commands when it is not known if the previous SCSI command was
processed is a potential data integrity risk.

For a sequential access device, consider the scenario in which a SCSI
SPACE command to backspace one filemark is issued and then reissued
due to no status received for the command. If the first SPACE
command was actually processed, the reissued SPACE command, if
processed, will cause the position to change. Thus, a subsequent
write operation will write data to the wrong position, and any
previous data at that position will be overwritten.

For a medium changer device, consider the scenario in which an
EXCHANGE MEDIUM command (the SOURCE ADDRESS and DESTINATION ADDRESS
are the same, thus performing a swap) is issued and then reissued due
to no status received for the command. If the first EXCHANGE MEDIUM
command was actually processed, the reissued EXCHANGE MEDIUM command,
if processed, will perform the swap again. The net effect is that no
swap was performed, thus putting data integrity at risk.

All commands that change the state of the device (e.g., SPACE
commands for sequential access devices and EXCHANGE MEDIUM commands
for medium changer devices) MUST be issued as non-immediate commands
for deterministic and ordered delivery to iSCSI targets.

For many of those state-changing commands, the execution model also
assumes that the command is executed exactly once. Devices
implementing READ POSITION and LOCATE provide a means for SCSI-level
command recovery, and new tape-class devices should support those
commands. In their absence, a retry at the SCSI level is difficult,
and error recovery at the iSCSI level is advisable.

10.6.1. Determining the Proper ErrorRecoveryLevel

The implementation and use of a specific ErrorRecoveryLevel should be
determined based on the deployment scenarios of a given iSCSI
implementation. Generally, the following factors must be considered
before deciding on the proper level of recovery:

a) Application resilience to I/O failures.

b) Required level of availability in the face of transport

connection failures.

c) Probability of transport-layer "checksum escape" (message error

undetected by TCP checksum -- see [RFC3385] for related
discussion). This in turn decides the iSCSI digest failure
frequency and thus the criticality of iSCSI-level error
recovery. The details of estimating this probability are
outside the scope of this document.

A consideration of the above factors for SCSI tape devices as an
example suggests that implementations SHOULD use ErrorRecoveryLevel=1
when transport connection failure is not a concern and SCSI-level
recovery is unavailable, and ErrorRecoveryLevel=2 when there is a
high likelihood of connection failure during a backup/retrieval.

For extended copy operations, implementations SHOULD use
ErrorRecoveryLevel=2 whenever there is a relatively high likelihood
of connection failure.

10.7. Multi-Task Abort Implementation Considerations

Multi-task abort operations are typically issued in emergencies, such
as clearing a device lock-up, HA failover/failback, etc. In these
circumstances, it is desirable to rapidly go through the error-
handling process as opposed to the target waiting on multiple third-
party initiators that may not even be functional anymore --
especially if this emergency is triggered because of one such
initiator failure. Therefore, both iSCSI target and initiator
implementations SHOULD support FastAbort multi-task abort semantics
(Section 4.2.3.4).

Note that in both standard semantics (Section 4.2.3.3) and FastAbort
semantics (Section 4.2.3.4) there may be outstanding data transfers
even after the TMF completion is reported on the issuing session. In
the case of iSCSI/iSER [RFC7145], these would be tagged data
transfers for STags not owned by any active tasks. Whether or not
real buffers support these data transfers is implementation
dependent. However, the data transfers logically MUST be silently
discarded by the target iSCSI layer in all cases. A target MAY, on
an implementation-defined internal timeout, also choose to drop the
connections on which it did not receive the expected Data-Out
sequences (Section 4.2.3.3) or NOP-Out acknowledgments
(Section 4.2.3.4) so as to reclaim the associated buffer, STag, and
TTT resources as appropriate.

11. iSCSI PDU Formats

All multi-byte integers that are specified in formats defined in this
document are to be represented in network byte order (i.e.,
big-endian). Any field that appears in this document assumes that
the most significant byte is the lowest numbered byte and the most
significant bit (within byte or field) is the lowest numbered bit
unless specified otherwise.

Any compliant sender MUST set all bits not defined and all reserved
fields to 0, unless specified otherwise. Any compliant receiver MUST
ignore any bit not defined and all reserved fields unless specified
otherwise. Receipt of reserved code values in defined fields MUST be
reported as a protocol error.

Reserved fields are marked by the word "reserved", some abbreviation
of "reserved", or by "." for individual bits when no other form of
marking is technically feasible.

11.1. iSCSI PDU Length and Padding

iSCSI PDUs are padded to the closest integer number of 4-byte words.
The padding bytes SHOULD be sent as 0.

11.2. PDU Template, Header, and Opcodes

All iSCSI PDUs have one or more header segments and, optionally, a
data segment. After the entire header segment group, a header digest
MAY follow. The data segment MAY also be followed by a data digest.

The Basic Header Segment (BHS) is the first segment in all of the
iSCSI PDUs. The BHS is a fixed-length 48-byte header segment. It
MAY be followed by Additional Header Segments (AHS), a Header-Digest,
a Data Segment, and/or a Data-Digest.

All PDU segments and digests are padded to the closest integer number
of 4-byte words. For example, all PDU segments and digests start at
a 4-byte word boundary, and the padding ranges from 0 to 3 bytes.
The padding bytes SHOULD be sent as 0.

iSCSI Response PDUs do not have AH Segments.

11.2.1. Basic Header Segment (BHS)

The BHS is 48 bytes long. The Opcode and DataSegmentLength fields
appear in all iSCSI PDUs. In addition, when used, the Initiator Task
Tag and Logical Unit Number always appear in the same location in the
header.

11.2.1.1. I (Immediate) Bit

For Request PDUs, the I bit set to 1 is an immediate delivery marker.

11.2.1.2. Opcode

The Opcode indicates the type of iSCSI PDU the header encapsulates.

The Opcodes are divided into two categories: initiator Opcodes and
target Opcodes. Initiator Opcodes are in PDUs sent by the initiator
(Request PDUs). Target Opcodes are in PDUs sent by the target
(Response PDUs).

Initiators MUST NOT use target Opcodes, and targets MUST NOT use
initiator Opcodes.

11.2.1.3. F (Final) Bit

11.2.1.4. Opcode-Specific Fields

11.2.1.5. TotalAHSLength

This is the total length of all AHS header segments in units of
4-byte words, including padding, if any.

The TotalAHSLength is only used in PDUs that have an AHS and MUST be
0 in all other PDUs.

11.2.1.6. DataSegmentLength

This is the data segment payload length in bytes (excluding padding).
The DataSegmentLength MUST be 0 whenever the PDU has no data segment.

11.2.1.7. LUN

Some Opcodes operate on a specific LU. The Logical Unit Number (LUN)
field identifies which LU. If the Opcode does not relate to a LU,
this field is either ignored or may be used in an Opcode-specific
way. The LUN field is 64 bits and should be formatted in accordance
with [SAM2]. For example, LUN[0] from [SAM2] is BHS byte 8 and so on
up to LUN[7] from [SAM2], which is BHS byte 15.

11.2.1.8. Initiator Task Tag

The initiator assigns a task tag to each iSCSI task it issues. While
a task exists, this tag MUST uniquely identify the task session-wide.
SCSI may also use the Initiator Task Tag as part of the SCSI task
identifier when the timespan during which an iSCSI Initiator Task Tag
must be unique extends over the timespan during which a SCSI task tag
must be unique. However, the iSCSI Initiator Task Tag must exist and
be unique even for untagged SCSI commands.

An ITT value of 0xffffffff is reserved and MUST NOT be assigned for a
task by the initiator. The only instance in which it may be seen on
the wire is in a target-initiated NOP-In PDU (Section 11.19) and in
the initiator response to that PDU, if necessary.

11.2.2.1. AHSType

The AHSType field is coded as follows:

bit 0-1 - Reserved

bit 2-7 - AHS code

- Reserved

- Extended CDB

- Bidirectional Read Expected Data Transfer Length

- 63 Reserved

11.2.2.2. AHSLength

This field contains the effective length in bytes of the AHS,
excluding AHSType and AHSLength and padding, if any. The AHS is
padded to the smallest integer number of 4-byte words (i.e., from 0
up to 3 padding bytes).

11.2.3. Header Digest and Data Digest

Optional header and data digests protect the integrity of the header
and data, respectively. The digests, if present, are located,
respectively, after the header and PDU-specific data and cover,
respectively, the header and the PDU data, each including the padding
bytes, if any.

The existence and type of digests are negotiated during the Login
Phase.

The separation of the header and data digests is useful in iSCSI
routing applications, in which only the header changes when a message
is forwarded. In this case, only the header digest should be
recalculated.

Digests are not included in data or header length fields.

A zero-length Data Segment also implies a zero-length Data-Digest.

11.2.4. Data Segment

The (optional) Data Segment contains PDU-associated data. Its
payload effective length is provided in the BHS field --
DataSegmentLength. The Data Segment is also padded to an integer
number of 4-byte words.

11.3.1. Flags and Task Attributes (Byte 1)

The flags for a SCSI Command PDU are:

bit 0 (F) is set to 1 when no unsolicited SCSI Data-Out PDUs
follow this PDU. When F = 1 for a write and if Expected
Data Transfer Length is larger than the
DataSegmentLength, the target may solicit additional data
through R2T.
bit 1 (R) is set to 1 when the command is expected to input
data.
bit 2 (W) is set to 1 when the command is expected to output
data.
bit 3-4 Reserved.
bit 5-7 contains Task Attributes.

Task Attributes (ATTR) have one of the following integer values (see
[SAM2] for details):

- Untagged

- Simple

- Ordered

3 - Head of queue

- ACA

5-7 - Reserved

At least one of the W and F bits MUST be set to 1.

Either or both of R and W MAY be 1 when the Expected Data Transfer
Length and/or the Bidirectional Read Expected Data Transfer Length
are 0, but they MUST NOT both be 0 when the Expected Data Transfer
Length and/or Bidirectional Read Expected Data Transfer Length are
not 0 (i.e., when some data transfer is expected, the transfer
direction is indicated by the R and/or W bit).

11.3.2. CmdSN - Command Sequence Number

The CmdSN enables ordered delivery across multiple connections in a
single session.

11.3.3. ExpStatSN

Command responses up to ExpStatSN - 1 (modulo 2**32) have been
received (acknowledges status) on the connection.

11.3.4. Expected Data Transfer Length

For unidirectional operations, the Expected Data Transfer Length
field contains the number of bytes of data involved in this SCSI
operation. For a unidirectional write operation (W flag set to 1 and
R flag set to 0), the initiator uses this field to specify the number
of bytes of data it expects to transfer for this operation. For a
unidirectional read operation (W flag set to 0 and R flag set to 1),
the initiator uses this field to specify the number of bytes of data
it expects the target to transfer to the initiator. It corresponds
to the SAM-2 byte count.

For bidirectional operations (both R and W flags are set to 1), this
field contains the number of data bytes involved in the write
transfer. For bidirectional operations, an additional header segment
MUST be present in the header sequence that indicates the
Bidirectional Read Expected Data Transfer Length. The Expected Data
Transfer Length field and the Bidirectional Read Expected Data
Transfer Length field correspond to the SAM-2 byte count.

If the Expected Data Transfer Length for a write and the length of
the immediate data part that follows the command (if any) are the
same, then no more data PDUs are expected to follow. In this case,
the F bit MUST be set to 1.

If the Expected Data Transfer Length is higher than the
FirstBurstLength (the negotiated maximum amount of unsolicited data
the target will accept), the initiator MUST send the maximum amount
of unsolicited data OR ONLY the immediate data, if any.

Upon completion of a data transfer, the target informs the initiator
(through residual counts) of how many bytes were actually processed
(sent and/or received) by the target.

11.3.5. CDB - SCSI Command Descriptor Block

There are 16 bytes in the CDB field to accommodate the commonly used
CDBs. Whenever the CDB is larger than 16 bytes, an Extended CDB AHS
MUST be used to contain the CDB spillover.

11.3.6. Data Segment - Command Data

Some SCSI commands require additional parameter data to accompany the
SCSI command. This data may be placed beyond the boundary of the
iSCSI header in a data segment. Alternatively, user data (e.g., from
a write operation) can be placed in the data segment (both cases are
referred to as immediate data). These data are governed by the rules
for solicited vs. unsolicited data outlined in Section 4.2.5.2.

11.4.1. Flags (Byte 1)

bit 1-2 Reserved.

bit 3 - (o) set for Bidirectional Read Residual Overflow. In this

case, the Bidirectional Read Residual Count indicates the
number of bytes that were not transferred to the
initiator because the initiator's Bidirectional Read
Expected Data Transfer Length was not sufficient.

bit 4 - (u) set for Bidirectional Read Residual Underflow. In this

case, the Bidirectional Read Residual Count indicates the
number of bytes that were not transferred to the
initiator out of the number of bytes expected to be
transferred.

bit 5 - (O) set for Residual Overflow. In this case, the Residual

Count indicates the number of bytes that were not
transferred because the initiator's Expected Data
Transfer Length was not sufficient. For a bidirectional
operation, the Residual Count contains the residual for
the write operation.

bit 6 - (U) set for Residual Underflow. In this case, the Residual

Count indicates the number of bytes that were not
transferred out of the number of bytes that were expected
to be transferred. For a bidirectional operation, the
Residual Count contains the residual for the write
operation.

bit 7 - (0) Reserved.

Bits O and U and bits o and u are mutually exclusive (i.e., having
both o and u or O and U set to 1 is a protocol error).

For a response other than "Command Completed at Target", bits 3-6
MUST be 0.

11.4.2. Status

The Status field is used to report the SCSI status of the command (as
specified in [SAM2]) and is only valid if the response code is
Command Completed at Target.

Some of the status codes defined in [SAM2] are:

0x00 GOOD
0x02 CHECK CONDITION
0x08 BUSY
0x18 RESERVATION CONFLICT

0x28 TASK SET FULL

0x30 ACA ACTIVE
0x40 TASK ABORTED

See [SAM2] for the complete list and definitions.

If a SCSI device error is detected while data from the initiator is
still expected (the command PDU did not contain all the data and the
target has not received a data PDU with the Final bit set), the
target MUST wait until it receives a data PDU with the F bit set in
the last expected sequence before sending the Response PDU.

11.4.3. Response

This field contains the iSCSI service response.

iSCSI service response codes defined in this specification are:

0x00 - Command Completed at Target

0x01 - Target Failure

0x80-0xff - Vendor specific

All other response codes are reserved.

The Response field is used to report a service response. The mapping
of the response code into a SCSI service response code value, if
needed, is outside the scope of this document. However, in symbolic
terms, response value 0x00 maps to the SCSI service response (see

[SAM2] and [SPC3]) of TASK COMPLETE or LINKED COMMAND COMPLETE. All
other Response values map to the SCSI service response of SERVICE
DELIVERY OR TARGET FAILURE.

If a SCSI Response PDU does not arrive before the session is
terminated, the SCSI service response is SERVICE DELIVERY OR TARGET
FAILURE.

A non-zero response field indicates a failure to execute the command,
in which case the Status and Flag fields are undefined and MUST be
ignored on reception.

11.4.4. SNACK Tag

This field contains a copy of the SNACK Tag of the last SNACK Tag
accepted by the target on the same connection and for the command for
which the response is issued. Otherwise, it is reserved and should
be set to 0.

After issuing a R-Data SNACK, the initiator must discard any SCSI
status unless contained in a SCSI Response PDU carrying the same
SNACK Tag as the last issued R-Data SNACK for the SCSI command on the
current connection.

For a detailed discussion on R-Data SNACK, see Section 11.16.3.

11.4.5. Residual Count

11.4.5.1. Field Semantics

The Residual Count field MUST be valid in the case where either the U
bit or the O bit is set. If neither bit is set, the Residual Count
field MUST be ignored on reception and SHOULD be set to 0 when
sending. Targets may set the residual count, and initiators may use
it when the response code is Command Completed at Target (even if the
status returned is not GOOD). If the O bit is set, the Residual
Count indicates the number of bytes that were not transferred because
the initiator's Expected Data Transfer Length was not sufficient. If
the U bit is set, the Residual Count indicates the number of bytes
that were not transferred out of the number of bytes expected to be
transferred.

11.4.5.2. Residuals Concepts Overview

"SCSI-Presented Data Transfer Length (SPDTL)" is the term this
document uses (see Section 2.2 for definition) to represent the
aggregate data length that the target SCSI layer attempts to transfer
using the local iSCSI layer for a task. "Expected Data Transfer

Length (EDTL)" is the iSCSI term that represents the length of data
that the iSCSI layer expects to transfer for a task. EDTL is
specified in the SCSI Command PDU.

When SPDTL = EDTL for a task, the target iSCSI layer completes the
task with no residuals. Whenever SPDTL differs from EDTL for a task,
that task is said to have a residual.

If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
SCSI Response PDU as specified in Section 11.4.5.1. The Residual
Count MUST be set to the numerical value of (SPDTL - EDTL).

If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
SCSI Response PDU as specified in Section 11.4.5.1. The Residual
Count MUST be set to the numerical value of (EDTL - SPDTL).

Note that the Overflow and Underflow scenarios are independent of
Data-In and Data-Out. Either scenario is logically possible in
either direction of data transfer.

11.4.5.3. SCSI REPORT LUNS Command and Residual Overflow

This section discusses the residual overflow issues, citing the
example of the SCSI REPORT LUNS command. Note, however, that there
are several SCSI commands (e.g., INQUIRY) with ALLOCATION LENGTH
fields following the same underlying rules. The semantics in the
rest of the section apply to all such SCSI commands.

The specification of the SCSI REPORT LUNS command requires that the
SCSI target limit the amount of data transferred to a maximum size
(ALLOCATION LENGTH) provided by the initiator in the REPORT LUNS CDB.

If the Expected Data Transfer Length (EDTL) in the iSCSI header of
the SCSI Command PDU for a REPORT LUNS command is set to at least as
large as that ALLOCATION LENGTH, the SCSI-layer truncation prevents
an iSCSI Residual Overflow from occurring. A SCSI initiator can
detect that such truncation has occurred via other information at the
SCSI layer. The rest of the section elaborates on this required
behavior.

The SCSI REPORT LUNS command requests a target SCSI layer to return a
LU inventory (LUN list) to the initiator SCSI layer (see Clause 6.21
of [SPC3]). The size of this LUN list may not be known to the
initiator SCSI layer when it issues the REPORT LUNS command; to avoid
transferring more LUN list data than the initiator is prepared for,
the REPORT LUNS CDB contains an ALLOCATION LENGTH field to specify
the maximum amount of data to be transferred to the initiator for
this command. If the initiator SCSI layer has underestimated the
number of LUs at the target, it is possible that the complete LU
inventory does not fit in the specified ALLOCATION LENGTH. In this
situation, Clause 4.3.4.6 of [SPC3] requires that the target SCSI
layer "shall terminate transfers to the Data-In Buffer" when the
number of bytes specified by the ALLOCATION LENGTH field have been
transferred.

Therefore, in response to a REPORT LUNS command, the SCSI layer at
the target presents at most ALLOCATION LENGTH bytes of data (LU
inventory) to iSCSI for transfer to the initiator. For a REPORT LUNS
command, if the iSCSI EDTL is at least as large as the ALLOCATION
LENGTH, the SCSI truncation ensures that the EDTL will accommodate
all of the data to be transferred. If all of the LU inventory data
presented to the iSCSI layer -- i.e., the data remaining after any
SCSI truncation -- is transferred to the initiator by the iSCSI
layer, an iSCSI Residual Overflow has not occurred and the iSCSI (O)
bit MUST NOT be set in the SCSI Response or final SCSI Data-Out PDU.
Note that this behavior is implied in Section 11.4.5.1, along with
the specification of the REPORT LUNS command in [SPC3]. However, if
the iSCSI EDTL is larger than the ALLOCATION LENGTH in this scenario,
note that the iSCSI Underflow MUST be signaled in the SCSI Response
PDU. An iSCSI Underflow MUST also be signaled when the iSCSI EDTL is
equal to the ALLOCATION LENGTH but the LU inventory data presented to
the iSCSI layer is smaller than the ALLOCATION LENGTH.

The LUN LIST LENGTH field in the LU inventory (the first field in the
inventory) is not affected by truncation of the inventory to fit in
ALLOCATION LENGTH; this enables a SCSI initiator to determine that
the received inventory is incomplete by noticing that the LUN LIST
LENGTH in the inventory is larger than the ALLOCATION LENGTH that was
sent in the REPORT LUNS CDB. A common initiator behavior in this
situation is to reissue the REPORT LUNS command with a larger
ALLOCATION LENGTH.

11.4.6. Bidirectional Read Residual Count

The Bidirectional Read Residual Count field MUST be valid in the case
where either the u bit or the o bit is set. If neither bit is set,
the Bidirectional Read Residual Count field is reserved. Targets may
set the Bidirectional Read Residual Count, and initiators may use it
when the response code is Command Completed at Target. If the o bit
is set, the Bidirectional Read Residual Count indicates the number of
bytes that were not transferred to the initiator because the
initiator's Bidirectional Read Expected Data Transfer Length was not
sufficient. If the u bit is set, the Bidirectional Read Residual
Count indicates the number of bytes that were not transferred to the
initiator out of the number of bytes expected to be transferred.

11.4.7. Data Segment - Sense and Response Data Segment

iSCSI targets MUST support and enable Autosense. If Status is CHECK
CONDITION (0x02), then the data segment MUST contain sense data for
the failed command.

For some iSCSI responses, the response data segment MAY contain some
response-related information (e.g., for a target failure, it may
contain a vendor-specific detailed description of the failure).

If the DataSegmentLength is not 0, the format of the data segment is
as follows:

The target reports the "Incorrect amount of data" condition if,
during data output, the total data length to output is greater than
FirstBurstLength and the initiator sent unsolicited non-immediate
data but the total amount of unsolicited data is different than
FirstBurstLength. The target reports the same error when the amount
of data sent as a reply to an R2T does not match the amount
requested.

11.4.8. ExpDataSN

This field indicates the number of Data-In (read) PDUs the target has
sent for the command.

This field MUST be 0 if the response code is not Command Completed at
Target or the target sent no Data-In PDUs for the command.

11.4.9. StatSN - Status Sequence Number

The StatSN is a sequence number that the target iSCSI layer generates
per connection and that in turn enables the initiator to acknowledge
status reception. The StatSN is incremented by 1 for every
response/status sent on a connection, except for responses sent as a
result of a retry or SNACK. In the case of responses sent due to a
retransmission request, the StatSN MUST be the same as the first time
the PDU was sent, unless the connection has since been restarted.

11.4.10. ExpCmdSN - Next Expected CmdSN from This Initiator

The ExpCmdSN is a sequence number that the target iSCSI returns to
the initiator to acknowledge command reception. It is used to update
a local variable with the same name. An ExpCmdSN equal to
MaxCmdSN + 1 indicates that the target cannot accept new commands.

11.4.11. MaxCmdSN - Maximum CmdSN from This Initiator

The MaxCmdSN is a sequence number that the target iSCSI returns to
the initiator to indicate the maximum CmdSN the initiator can send.
It is used to update a local variable with the same name. If the
MaxCmdSN is equal to ExpCmdSN - 1, this indicates to the initiator
that the target cannot receive any additional commands. When the
MaxCmdSN changes at the target while the target has no pending PDUs
to convey this information to the initiator, it MUST generate a
NOP-In to carry the new MaxCmdSN.

11.5.1. Function

The task management functions provide an initiator with a way to
explicitly control the execution of one or more tasks (SCSI and iSCSI
tasks). The task management function codes are listed below. For a
more detailed description of SCSI task management, see [SAM2].

Values 9-12 are assigned in [RFC7144]. All other possible values for
the Function field are unassigned.

For all these functions, the Task Management Function Response MUST
be returned as detailed in Section 11.6. All these functions apply
to the referenced tasks, regardless of whether they are proper SCSI
tasks or tagged iSCSI operations. Task management requests must act
on all the commands from the same session having a CmdSN lower than
the task management CmdSN. LOGICAL UNIT RESET, TARGET WARM RESET,
and TARGET COLD RESET may affect commands from other sessions or
commands from the same session, regardless of their CmdSN value.

If the task management request is marked for immediate delivery, it
must be considered immediately for execution, but the operations
involved (all or part of them) may be postponed to allow the target
to receive all relevant tasks. According to [SAM2], for all the
tasks covered by the task management response (i.e., with a CmdSN
lower than the task management command CmdSN), except for the task
management response to a TASK REASSIGN, additional responses MUST NOT
be delivered to the SCSI layer after the task management response.
The iSCSI initiator MAY deliver to the SCSI layer all responses
received before the task management response (i.e., it is a matter of
implementation if the SCSI responses that are received before the
task management response but after the task management request was
issued are delivered to the SCSI layer by the iSCSI layer in the
initiator). The iSCSI target MUST ensure that no responses for the
tasks covered by a task management function are delivered to the
iSCSI initiator after the task management response, except for a task
covered by a TASK REASSIGN.

For ABORT TASK SET and CLEAR TASK SET, the issuing initiator MUST
continue to respond to all valid Target Transfer Tags (received via
R2T, Text Response, NOP-In, or SCSI Data-In PDUs) related to the
affected task set, even after issuing the task management request.

The issuing initiator SHOULD, however, terminate (i.e., by setting
the F bit to 1) these response sequences as quickly as possible. The
target for its part MUST wait for responses on all affected Target
Transfer Tags before acting on either of these two task management
requests. If all or part of the response sequence is not received
(due to digest errors) for a valid TTT, the target MAY treat it as a
case of a within-command error recovery class (see Section 7.1.4.1)
if it is supporting ErrorRecoveryLevel >= 1 or, alternatively, may
drop the connection to complete the requested task set function.

If an ABORT TASK is issued for a task created by an immediate
command, then the RefCmdSN MUST be that of the task management
request itself (i.e., the CmdSN and RefCmdSN are equal); otherwise,
the RefCmdSN MUST be set to the CmdSN of the task to be aborted
(lower than the CmdSN).

If the connection is still active (i.e., it is not undergoing an
implicit or explicit logout), an ABORT TASK MUST be issued on the
same connection to which the task to be aborted is allegiant at the
time the task management request is issued. If the connection is
implicitly or explicitly logged out (i.e., no other request will be
issued on the failing connection and no other response will be
received on the failing connection), then an ABORT TASK function
request may be issued on another connection. This task management
request will then establish a new allegiance for the command to be
aborted as well as abort it (i.e., the task to be aborted will not
have to be retried or reassigned, and its status, if sent but not
acknowledged, will be resent followed by the task management
response).

At the target, an ABORT TASK function MUST NOT be executed on a task
management request; such a request MUST result in a task management
response of "Function rejected".

For the LOGICAL UNIT RESET function, the target MUST behave as
dictated by the Logical Unit Reset function in [SAM2].

The implementation of the TARGET WARM RESET function and the TARGET
COLD RESET function is OPTIONAL and, when implemented, should act as
described below. The TARGET WARM RESET is also subject to SCSI
access controls on the requesting initiator as defined in [SPC3].
When authorization fails at the target, the appropriate response as
described in Section 11.6.1 MUST be returned by the target. The
TARGET COLD RESET function is not subject to SCSI access controls,
but its execution privileges may be managed by iSCSI mechanisms such
as login authentication.

When executing the TARGET WARM RESET and TARGET COLD RESET functions,
the target cancels all pending operations on all LUs known by the
issuing initiator. Both functions are equivalent to the TARGET RESET
function specified by [SAM2]. They can affect many other initiators
logged in with the servicing SCSI target port.

Additionally, the target MUST treat the TARGET COLD RESET function as
a power-on event, thus terminating all of its TCP connections to all
initiators (all sessions are terminated). For this reason, the
service response (defined by [SAM2]) for this SCSI task management
function may not be reliably delivered to the issuing initiator port.

For the TASK REASSIGN function, the target should reassign the
connection allegiance to this new connection (and thus resume iSCSI
exchanges for the task). TASK REASSIGN MUST ONLY be received by the
target after the connection on which the command was previously
executing has been successfully logged out. The task management
response MUST be issued before the reassignment becomes effective.

For additional usage semantics, see Section 7.2.

At the target, a TASK REASSIGN function request MUST NOT be executed
to reassign the connection allegiance of a Task Management Function
Request, an active text negotiation task, or a Logout task; such a
request MUST result in a task management response of "Function
rejected".

TASK REASSIGN MUST be issued as an immediate command.

11.5.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.5.3. LUN

This field is required for functions that address a specific LU
(ABORT TASK, CLEAR TASK SET, ABORT TASK SET, CLEAR ACA, LOGICAL UNIT
RESET) and is reserved in all others.

11.5.4. Referenced Task Tag

This is the Initiator Task Tag of the task to be aborted for the
ABORT TASK function or reassigned for the TASK REASSIGN function.
For all the other functions, this field MUST be set to the reserved
value 0xffffffff.

11.5.5. RefCmdSN

If an ABORT TASK is issued for a task created by an immediate
command, then the RefCmdSN MUST be that of the task management
request itself (i.e., the CmdSN and RefCmdSN are equal).

For an ABORT TASK of a task created by a non-immediate command, the
RefCmdSN MUST be set to the CmdSN of the task identified by the
Referenced Task Tag field. Targets must use this field as described
in Section 11.6.1 when the task identified by the Referenced Task Tag
field is not with the target.

Otherwise, this field is reserved.

11.5.6. ExpDataSN

For recovery purposes, the iSCSI target and initiator maintain a data
acknowledgment reference number -- the first input DataSN number
unacknowledged by the initiator. When issuing a new command, this
number is set to 0. If the function is TASK REASSIGN, which
establishes a new connection allegiance for a previously issued read
or bidirectional command, the ExpDataSN will contain an updated data
acknowledgment reference number or the value 0; the latter indicates
that the data acknowledgment reference number is unchanged. The
initiator MUST discard any data PDUs from the previous execution that
it did not acknowledge, and the target MUST transmit all Data-In PDUs
(if any) starting with the data acknowledgment reference number. The
number of retransmitted PDUs may or may not be the same as the
original transmission, depending on if there was a change in
MaxRecvDataSegmentLength in the reassignment. The target MAY also
send no more Data-In PDUs if all data has been acknowledged.

The value of ExpDataSN MUST be 0 or higher than the DataSN of the
last acknowledged Data-In PDU, but not larger than DataSN + 1 of the
last Data-IN PDU sent by the target. Any other value MUST be ignored
by the target.

For the functions ABORT TASK, ABORT TASK SET, CLEAR ACA, CLEAR TASK
SET, LOGICAL UNIT RESET, TARGET COLD RESET, TARGET WARM RESET, and
TASK REASSIGN, the target performs the requested task management
function and sends a task management response back to the initiator.
For TASK REASSIGN, the new connection allegiance MUST ONLY become
effective at the target after the target issues the task management
response.

11.6.1. Response

The target provides a response, which may take on the following
values:

For a discussion on the usage of response codes 3 and 4, see
Section 7.2.2.

For the TARGET COLD RESET and TARGET WARM RESET functions, the target
cancels all pending operations across all LUs known to the issuing
initiator. For the TARGET COLD RESET function, the target MUST then
close all of its TCP connections to all initiators (terminates all
sessions).

The mapping of the response code into a SCSI service response code
value, if needed, is outside the scope of this document. However, in
symbolic terms, Response values 0 and 1 map to the SCSI service
response of FUNCTION COMPLETE. Response value 2 maps to the SCSI
service response of INCORRECT LOGICAL UNIT NUMBER. All other
Response values map to the SCSI service response of FUNCTION
REJECTED. If a Task Management Function Response PDU does not arrive
before the session is terminated, the SCSI service response is
SERVICE DELIVERY OR TARGET FAILURE.

The response to ABORT TASK SET and CLEAR TASK SET MUST only be issued
by the target after all of the commands affected have been received
by the target, the corresponding task management functions have been
executed by the SCSI target, and the delivery of all responses
delivered until the task management function completion has been
confirmed (acknowledged through the ExpStatSN) by the initiator on
all connections of this session. For the exact timeline of events,
refer to Sections 4.2.3.3 and 4.2.3.4.

For the ABORT TASK function,

a) if the Referenced Task Tag identifies a valid task leading to a

successful termination, then targets must return the "Function
complete" response.

b) if the Referenced Task Tag does not identify an existing task

but the CmdSN indicated by the RefCmdSN field in the Task
Management Function Request is within the valid CmdSN window
and less than the CmdSN of the Task Management Function Request
itself, then targets must consider the CmdSN as received and
return the "Function complete" response.

c) if the Referenced Task Tag does not identify an existing task

and the CmdSN indicated by the RefCmdSN field in the Task
Management Function Request is outside the valid CmdSN window,
then targets must return the "Task does not exist" response.

For response semantics on function types that can potentially impact
multiple active tasks on the target, see Section 4.2.3.

Status can accompany the last Data-In PDU if the command did not end
with an exception (i.e., the status is "good status" -- GOOD,
CONDITION MET, or INTERMEDIATE-CONDITION MET). The presence of
status (and of a residual count) is signaled via the S flag bit.
Although targets MAY choose to send even non-exception status in
separate responses, initiators MUST support non-exception status in
Data-In PDUs.

11.7.1. F (Final) Bit

For outgoing data, this bit is 1 for the last PDU of unsolicited data
or the last PDU of a sequence that answers an R2T.

For incoming data, this bit is 1 for the last input (read) data PDU
of a sequence. Input can be split into several sequences, each
having its own F bit. Splitting the data stream into sequences does
not affect DataSN counting on Data-In PDUs. It MAY be used as a
"change direction" indication for bidirectional operations that need
such a change.

DataSegmentLength MUST NOT exceed MaxRecvDataSegmentLength for the
direction it is sent, and the total of all the DataSegmentLength of
all PDUs in a sequence MUST NOT exceed MaxBurstLength (or
FirstBurstLength for unsolicited data). However, the number of
individual PDUs in a sequence (or in total) may be higher than the
ratio of MaxBurstLength (or FirstBurstLength) to
MaxRecvDataSegmentLength (as PDUs may be limited in length by the
capabilities of the sender). Using a DataSegmentLength of 0 may
increase beyond what is reasonable for the number of PDUs and should
therefore be avoided.

For bidirectional operations, the F bit is 1 for both the end of the
input sequences and the end of the output sequences.

11.7.2. A (Acknowledge) Bit

For sessions with ErrorRecoveryLevel=1 or higher, the target sets
this bit to 1 to indicate that it requests a positive acknowledgment
from the initiator for the data received. The target should use the
A bit moderately; it MAY only set the A bit to 1 once every
MaxBurstLength bytes, or on the last Data-In PDU that concludes the
entire requested read data transfer for the task from the target's
perspective, and it MUST NOT do so more frequently. The target MUST
NOT set to 1 the A bit for sessions with ErrorRecoveryLevel=0. The
initiator MUST ignore the A bit set to 1 for sessions with
ErrorRecoveryLevel=0.

On receiving a Data-In PDU with the A bit set to 1 on a session with
ErrorRecoveryLevel greater than 0, if there are no holes in the read
data until that Data-In PDU, the initiator MUST issue a SNACK of type
DataACK, except when it is able to acknowledge the status for the
task immediately via the ExpStatSN on other outbound PDUs if the
status for the task is also received. In the latter case
(acknowledgment through the ExpStatSN), sending a SNACK of type
DataACK in response to the A bit is OPTIONAL, but if it is done, it
must not be sent after the status acknowledgment through the

ExpStatSN. If the initiator has detected holes in the read data
prior to that Data-In PDU, it MUST postpone issuing the SNACK of type
DataACK until the holes are filled. An initiator also MUST NOT
acknowledge the status for the task before those holes are filled. A
status acknowledgment for a task that generated the Data-In PDUs is
considered by the target as an implicit acknowledgment of the Data-In
PDUs if such an acknowledgment was requested by the target.

11.7.3. Flags (Byte 1)

The last SCSI data packet sent from a target to an initiator for a
SCSI command that completed successfully (with a status of GOOD,
CONDITION MET, INTERMEDIATE, or INTERMEDIATE-CONDITION MET) may also
optionally contain the Status for the data transfer. In this case,
Sense Data cannot be sent together with the Command Status. If the
command is completed with an error, then the response and sense data
MUST be sent in a SCSI Response PDU (i.e., MUST NOT be sent in a SCSI
data packet). For bidirectional commands, the status MUST be sent in
a SCSI Response PDU.

bit 2-4 - Reserved.
bit 5-6 - used the same as in a SCSI Response. These
bits are only valid when S is set to 1. For
details, see Section 11.4.1.

bit 7 S (status) - set to indicate that the Command Status field

contains status. If this bit is set to 1, the
F bit MUST also be set to 1.

The fields StatSN, Status, and Residual Count only have meaningful
content if the S bit is set to 1. The values for these fields are
defined in Section 11.4.

11.7.4. Target Transfer Tag and LUN

On outgoing data, the Target Transfer Tag is provided to the target
if the transfer is honoring an R2T. In this case, the Target
Transfer Tag field is a replica of the Target Transfer Tag provided
with the R2T.

On incoming data, the Target Transfer Tag and LUN MUST be provided by
the target if the A bit is set to 1; otherwise, they are reserved.
The Target Transfer Tag and LUN are copied by the initiator into the
SNACK of type DataACK that it issues as a result of receiving a SCSI
Data-In PDU with the A bit set to 1.

The Target Transfer Tag values are not specified by this protocol,
except that the value 0xffffffff is reserved and means that the
Target Transfer Tag is not supplied. If the Target Transfer Tag is
provided, then the LUN field MUST hold a valid value and be
consistent with whatever was specified with the command; otherwise,
the LUN field is reserved.

11.7.5. DataSN

For input (read) or bidirectional Data-In PDUs, the DataSN is the
input PDU number within the data transfer for the command identified
by the Initiator Task Tag.

For output (write) data PDUs, the DataSN is the Data-Out PDU number
within the current output sequence. Either the current output
sequence is identified by the Initiator Task Tag (for unsolicited
data) or it is a data sequence generated for one R2T (for data
solicited through R2T).

11.7.6. Buffer Offset

The Buffer Offset field contains the offset of this PDU payload data
within the complete data transfer. The sum of the buffer offset and
length should not exceed the expected transfer length for the
command.

The order of data PDUs within a sequence is determined by
DataPDUInOrder. When set to Yes, it means that PDUs have to be in
increasing buffer offset order and overlays are forbidden.

The ordering between sequences is determined by DataSequenceInOrder.
When set to Yes, it means that sequences have to be in increasing
buffer offset order and overlays are forbidden.

11.7.7. DataSegmentLength

This is the data payload length of a SCSI Data-In or SCSI Data-Out
PDU. The sending of 0-length data segments should be avoided, but
initiators and targets MUST be able to properly receive 0-length data
segments.

The data segments of Data-In and Data-Out PDUs SHOULD be filled to
the integer number of 4-byte words (real payload), unless the F bit
is set to 1.

When an initiator has submitted a SCSI command with data that passes
from the initiator to the target (write), the target may specify
which blocks of data it is ready to receive. The target may request
that the data blocks be delivered in whichever order is convenient
for the target at that particular instant. This information is
passed from the target to the initiator in the Ready To Transfer
(R2T) PDU.

In order to allow write operations without an explicit initial R2T,
the initiator and target MUST have negotiated the key InitialR2T to
No during login.

An R2T MAY be answered with one or more SCSI Data-Out PDUs with a
matching Target Transfer Tag. If an R2T is answered with a single
Data-Out PDU, the buffer offset in the data PDU MUST be the same as
the one specified by the R2T, and the data length of the data PDU
MUST be the same as the Desired Data Transfer Length specified in the
R2T. If the R2T is answered with a sequence of data PDUs, the buffer
offset and length MUST be within the range of those specified by the
R2T, and the last PDU MUST have the F bit set to 1. If the last PDU
(marked with the F bit) is received before the Desired Data Transfer
Length is transferred, a target MAY choose to reject that PDU with
the "Protocol Error" reason code. DataPDUInOrder governs the
Data-Out PDU ordering. If DataPDUInOrder is set to Yes, the buffer
offsets and lengths for consecutive PDUs MUST form a continuous
non-overlapping range, and the PDUs MUST be sent in increasing offset
order.

The target may send several R2T PDUs. It therefore can have a number
of pending data transfers. The number of outstanding R2T PDUs is
limited by the value of the negotiated key MaxOutstandingR2T. Within
a task, outstanding R2Ts MUST be fulfilled by the initiator in the
order in which they were received.

R2T PDUs MAY also be used to recover Data-Out PDUs. Such an R2T
(Recovery-R2T) is generated by a target upon detecting the loss of
one or more Data-Out PDUs due to:

Digest error

Sequence error

Sequence reception timeout

A Recovery-R2T carries the next unused R2TSN but requests part of or
the entire data burst that an earlier R2T (with a lower R2TSN) had
already requested.

DataSequenceInOrder governs the buffer offset ordering in consecutive
R2Ts. If DataSequenceInOrder is Yes, then consecutive R2Ts MUST
refer to continuous non-overlapping ranges, except for Recovery-R2Ts.

11.8.1. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.8.2. R2TSN

R2TSN is the R2T PDU input PDU number within the command identified
by the Initiator Task Tag.

11.8.3. StatSN

The StatSN field will contain the next StatSN. The StatSN for this
connection is not advanced after this PDU is sent.

11.8.4. Desired Data Transfer Length and Buffer Offset

The target specifies how many bytes it wants the initiator to send
because of this R2T PDU. The target may request the data from the
initiator in several chunks, not necessarily in the original order of
the data. The target therefore also specifies a buffer offset that
indicates the point at which the data transfer should begin, relative
to the beginning of the total data transfer. The Desired Data
Transfer Length MUST NOT be 0 and MUST NOT exceed MaxBurstLength.

11.8.5. Target Transfer Tag

The target assigns its own tag to each R2T request that it sends to
the initiator. This tag can be used by the target to easily identify
the data it receives. The Target Transfer Tag and LUN are copied in
the outgoing data PDUs and are only used by the target. There is no
protocol rule about the Target Transfer Tag except that the value
0xffffffff is reserved and MUST NOT be sent by a target in an R2T.

11.9. Asynchronous Message

An Asynchronous Message may be sent from the target to the initiator
without corresponding to a particular command. The target specifies
the reason for the event and sense data.

Some Asynchronous Messages are strictly related to iSCSI, while
others are related to SCSI [SAM2].

The StatSN counts this PDU as an acknowledgeable event (the StatSN is
advanced), which allows for initiator and target state
synchronization.

11.9.1. AsyncEvent

The codes used for iSCSI Asynchronous Messages (events) are:

(SCSI Async Event) - a SCSI asynchronous event is reported in
the sense data. Sense Data that accompanies the report, in
the data segment, identifies the condition. The sending of a
SCSI event ("asynchronous event reporting" in SCSI
terminology) is dependent on the target support for SCSI
asynchronous event reporting (see [SAM2]) as indicated in the
standard INQUIRY data (see [SPC3]). Its use may be enabled by
parameters in the SCSI Control mode page (see [SPC3]).

(Logout Request) - the target requests Logout. This Async
Message MUST be sent on the same connection as the one
requesting to be logged out. The initiator MUST honor this
request by issuing a Logout as early as possible but no later
than Parameter3 seconds. The initiator MUST send a Logout
with a reason code of "close the connection" OR "close the
session" to close all the connections. Once this message is
received, the initiator SHOULD NOT issue new iSCSI commands on
the connection to be logged out. The target MAY reject any
new I/O requests that it receives after this message with the
reason code "Waiting for Logout". If the initiator does not
log out in Parameter3 seconds, the target should send an Async
PDU with iSCSI event code "Dropped the connection" if possible
or simply terminate the transport connection. Parameter1 and
Parameter2 are reserved.

(Connection Drop Notification) - the target indicates that it
will drop the connection.

The Parameter1 field indicates the CID of the connection that
is going to be dropped.

The Parameter2 field (Time2Wait) indicates, in seconds, the
minimum time to wait before attempting to reconnect or
reassign.

The Parameter3 field (Time2Retain) indicates the maximum time
allowed to reassign commands after the initial wait (in
Parameter2).

If the initiator does not attempt to reconnect and/or reassign
the outstanding commands within the time specified by
Parameter3, or if Parameter3 is 0, the target will terminate
all outstanding commands on this connection. In this case, no
other responses should be expected from the target for the
outstanding commands on this connection.

A value of 0 for Parameter2 indicates that reconnect can be
attempted immediately.

3 (Session Drop Notification) - the target indicates that it

will drop all the connections of this session.

The Parameter1 field is reserved.

The Parameter2 field (Time2Wait) indicates, in seconds, the
minimum time to wait before attempting to reconnect.

The Parameter3 field (Time2Retain) indicates the maximum time
allowed to reassign commands after the initial wait (in
Parameter2).

If the initiator does not attempt to reconnect and/or reassign
the outstanding commands within the time specified by
Parameter3, or if Parameter3 is 0, the session is terminated.
In this case, the target will terminate all outstanding
commands in this session; no other responses should be
expected from the target for the outstanding commands in this
session. A value of 0 for Parameter2 indicates that reconnect
can be attempted immediately.

(Negotiation Request) - the target requests parameter
negotiation on this connection. The initiator MUST honor this
request by issuing a Text Request (that can be empty) on the
same connection as early as possible, but no later than
Parameter3 seconds, unless a Text Request is already pending
on the connection, or by issuing a Logout Request. If the
initiator does not issue a Text Request, the target may
reissue the Asynchronous Message requesting parameter
negotiation.

(Task Termination) - all active tasks for a LU with a matching
LUN field in the Async Message PDU are being terminated. The
receiving initiator iSCSI layer MUST respond to this message
by taking the following steps, in order:

Stop Data-Out transfers on that connection for all active
TTTs for the affected LUN quoted in the Async Message PDU.

Acknowledge the StatSN of the Async Message PDU via a
NOP-Out PDU with ITT=0xffffffff (i.e., non-ping flavor),
while copying the LUN field from the Async Message to
NOP-Out.

This value of AsyncEvent, however, MUST NOT be used on an
iSCSI session unless the new TaskReporting text key defined in
Section 13.23 was negotiated to FastAbort on the session.

An initiator MUST NOT have more than one outstanding Text Request on
a connection at any given time.

On a connection failure, an initiator must either explicitly abort
any active allegiant text negotiation task or cause such a task to be
implicitly terminated by the target.

11.10.1. F (Final) Bit

When set to 1, this bit indicates that this is the last or only Text
Request in a sequence of Text Requests; otherwise, it indicates that
more Text Requests will follow.

11.10.2. C (Continue) Bit

When set to 1, this bit indicates that the text (set of key=value
pairs) in this Text Request is not complete (it will be continued on
subsequent Text Requests); otherwise, it indicates that this Text
Request ends a set of key=value pairs. A Text Request with the C bit
set to 1 MUST have the F bit set to 0.

11.10.3. Initiator Task Tag

This is the initiator-assigned identifier for this Text Request. If
the command is sent as part of a sequence of Text Requests and
responses, the Initiator Task Tag MUST be the same for all the
requests within the sequence (similar to linked SCSI commands). The
I bit for all requests in a sequence also MUST be the same.

11.10.4. Target Transfer Tag

When the Target Transfer Tag is set to the reserved value 0xffffffff,
it tells the target that this is a new request, and the target resets
any internal state associated with the Initiator Task Tag (resets the
current negotiation state).

The target sets the Target Transfer Tag in a Text Response to a value
other than the reserved value 0xffffffff whenever it indicates that
it has more data to send or more operations to perform that are
associated with the specified Initiator Task Tag. It MUST do so
whenever it sets the F bit to 0 in the response. By copying the
Target Transfer Tag from the response to the next Text Request, the
initiator tells the target to continue the operation for the specific
Initiator Task Tag. The initiator MUST ignore the Target Transfer
Tag in the Text Response when the F bit is set to 1.

This mechanism allows the initiator and target to transfer a large
amount of textual data over a sequence of text-command/text-response
exchanges or to perform extended negotiation sequences.

If the Target Transfer Tag is not 0xffffffff, the LUN field MUST be
sent by the target in the Text Response.

A target MAY reset its internal negotiation state if an exchange is
stalled by the initiator for a long time or if it is running out of
resources.

Long Text Responses are handled as shown in the following example:

I->T Text SendTargets=All (F = 1, TTT = 0xffffffff)

T->I Text <part 1> (F = 0, TTT = 0x12345678)

I->T Text <empty> (F = 1, TTT = 0x12345678)

T->I Text <part 2> (F = 0, TTT = 0x12345678)

I->T Text <empty> (F = 1, TTT = 0x12345678)

...

T->I Text <part n> (F = 1, TTT = 0xffffffff)

11.10.5. Text

The data lengths of a Text Request MUST NOT exceed the iSCSI target
MaxRecvDataSegmentLength (a parameter that is negotiated per
connection and per direction). The text format is specified in
Section 6.2.

Sections 12 and 13 list some basic Text key=value pairs, some of
which can be used in Login Requests/Responses and some in Text
Requests/Responses.

A key=value pair can span Text Request or Text Response boundaries.
A key=value pair can start in one PDU and continue on the next. In
other words, the end of a PDU does not necessarily signal the end of
a key=value pair.

The target responds by sending its response back to the initiator.
The response text format is similar to the request text format. The
Text Response MAY refer to key=value pairs presented in an earlier
Text Request, and the text in the request may refer to earlier
responses.

Section 6.2 details the rules for the Text Requests and Responses.

Text operations are usually meant for parameter setting/negotiations
but can also be used to perform some long-lasting operations.

Text operations that take a long time should be placed in their own
Text Request.

11.11. Text Response

The Text Response PDU contains the target's responses to the
initiator's Text Request. The format of the Text field matches that
of the Text Request.

11.11.1. F (Final) Bit

When set to 1, in response to a Text Request with the Final bit set
to 1, the F bit indicates that the target has finished the whole
operation. Otherwise, if set to 0 in response to a Text Request with
the Final Bit set to 1, it indicates that the target has more work to
do (invites a follow-on Text Request). A Text Response with the
F bit set to 1 in response to a Text Request with the F bit set to 0
is a protocol error.

A Text Response with the F bit set to 1 MUST NOT contain key=value
pairs that may require additional answers from the initiator.

A Text Response with the F bit set to 1 MUST have a Target Transfer
Tag field set to the reserved value 0xffffffff.

A Text Response with the F bit set to 0 MUST have a Target Transfer
Tag field set to a value other than the reserved value 0xffffffff.

11.11.2. C (Continue) Bit

When set to 1, this bit indicates that the text (set of key=value
pairs) in this Text Response is not complete (it will be continued on
subsequent Text Responses); otherwise, it indicates that this Text
Response ends a set of key=value pairs. A Text Response with the
C bit set to 1 MUST have the F bit set to 0.

11.11.3. Initiator Task Tag

The Initiator Task Tag matches the tag used in the initial Text
Request.

11.11.4. Target Transfer Tag

When a target has more work to do (e.g., cannot transfer all the
remaining text data in a single Text Response or has to continue the
negotiation) and has enough resources to proceed, it MUST set the
Target Transfer Tag to a value other than the reserved value
0xffffffff. Otherwise, the Target Transfer Tag MUST be set to
0xffffffff.

When the Target Transfer Tag is not 0xffffffff, the LUN field may be
significant.

The initiator MUST copy the Target Transfer Tag and LUN in its next
request to indicate that it wants the rest of the data.

When the target receives a Text Request with the Target Transfer Tag
set to the reserved value 0xffffffff, it resets its internal
information (resets state) associated with the given Initiator Task
Tag (restarts the negotiation).

When a target cannot finish the operation in a single Text Response
and does not have enough resources to continue, it rejects the Text
Request with the appropriate Reject code.

A target may reset its internal state associated with an Initiator
Task Tag (the current negotiation state) as expressed through the
Target Transfer Tag if the initiator fails to continue the exchange
for some time. The target may reject subsequent Text Requests with
the Target Transfer Tag set to the "stale" value.

11.11.5. StatSN

The target StatSN variable is advanced by each Text Response sent.

11.11.6. Text Response Data

The data lengths of a Text Response MUST NOT exceed the iSCSI
initiator MaxRecvDataSegmentLength (a parameter that is negotiated
per connection and per direction).

The text in the Text Response Data is governed by the same rules as
the text in the Text Request Data (see Section 11.11.2).

Although the initiator is the requesting party and controls the
request-response initiation and termination, the target can offer
key=value pairs of its own as part of a sequence and not only in
response to the initiator.

11.12. Login Request

After establishing a TCP connection between an initiator and a
target, the initiator MUST start a Login Phase to gain further access
to the target's resources.

The Login Phase (see Section 6.3) consists of a sequence of Login
Requests and Login Responses that carry the same Initiator Task Tag.

11.12.1. T (Transit) Bit

When set to 1, this bit indicates that the initiator is ready to
transit to the next stage.

If the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
this also indicates that the initiator is ready for the Login
Final-Response (see Section 6.3).

11.12.2. C (Continue) Bit

When set to 1, this bit indicates that the text (set of key=value
pairs) in this Login Request is not complete (it will be continued on
subsequent Login Requests); otherwise, it indicates that this Login
Request ends a set of key=value pairs. A Login Request with the
C bit set to 1 MUST have the T bit set to 0.

11.12.3. CSG and NSG

Through these fields -- Current Stage (CSG) and Next Stage (NSG) --
the Login negotiation requests and responses are associated with a
specific stage in the session (SecurityNegotiation,
LoginOperationalNegotiation, FullFeaturePhase) and may indicate the
next stage to which they want to move (see Section 6.3). The Next
Stage value is only valid when the T bit is 1; otherwise, it is
reserved.

The stage codes are:

- SecurityNegotiation

- LoginOperationalNegotiation

- FullFeaturePhase

All other codes are reserved.

11.12.4. Version

The version number for this document is 0x00. Therefore, both
Version-min and Version-max MUST be set to 0x00.

11.12.4.1. Version-max

Version-max indicates the maximum version number supported.

All Login Requests within the Login Phase MUST carry the same
Version-max.

The target MUST use the value presented with the first Login Request.

11.12.4.2. Version-min

All Login Requests within the Login Phase MUST carry the same
Version-min. The target MUST use the value presented with the first
Login Request.

11.12.5. ISID

This is an initiator-defined component of the session identifier and
is structured as follows (see Section 10.1.1 for details):

For the T field values 00b and 01b, a combination of A and B (for
00b) or B and C (for 01b) identifies the vendor or organization whose
component (software or hardware) generates this ISID. A vendor or
organization with one or more OUIs, or one or more Enterprise
Numbers, MUST use at least one of these numbers and select the
appropriate value for the T field when its components generate ISIDs.
An OUI or EN MUST be set in the corresponding fields in network byte
order (byte big-endian).

If the T field is 10b, B and C are set to a random 24-bit unsigned
integer value in network byte order (byte big-endian). See [RFC3721]
for how this affects the principle of "conservative reuse".

The Qualifier field is a 16-bit or 24-bit unsigned integer value that
provides a range of possible values for the ISID within the selected
namespace. It may be set to any value within the constraints
specified in the iSCSI protocol (see Sections 4.4.3 and 10.1.1).

The T field value of 11b is reserved.

If the ISID is derived from something assigned to a hardware adapter
or interface by a vendor as a preset default value, it MUST be
configurable to a value assigned according to the SCSI port behavior
desired by the system in which it is installed (see Sections 10.1.1
and 10.1.2). The resultant ISID MUST also be persistent over power
cycles, reboot, card swap, etc.

11.12.6. TSIH

The TSIH must be set in the first Login Request. The reserved value
0 MUST be used on the first connection for a new session. Otherwise,
the TSIH sent by the target at the conclusion of the successful login
of the first connection for this session MUST be used. The TSIH
identifies to the target the associated existing session for this new
connection.

All Login Requests within a Login Phase MUST carry the same TSIH.

The target MUST check the value presented with the first Login
Request and act as specified in Section 6.3.1.

11.12.7. Connection ID (CID)

The CID provides a unique ID for this connection within the session.

All Login Requests within the Login Phase MUST carry the same CID.

The target MUST use the value presented with the first Login Request.

A Login Request with a non-zero TSIH and a CID equal to that of an
existing connection implies a logout of the connection followed by a
login (see Section 6.3.4). For details regarding the implicit Logout
Request, see Section 11.14.

11.12.8. CmdSN

The CmdSN is either the initial command sequence number of a session
(for the first Login Request of a session -- the "leading" login) or
the command sequence number in the command stream if the login is for
a new connection in an existing session.

Examples:

Login on a leading connection: If the leading login carries the
CmdSN 123, all other Login Requests in the same Login Phase carry
the CmdSN 123, and the first non-immediate command in the Full
Feature Phase also carries the CmdSN 123.

Login on other than a leading connection: If the current CmdSN at
the time the first login on the connection is issued is 500, then
that PDU carries CmdSN=500. Subsequent Login Requests that are
needed to complete this Login Phase may carry a CmdSN higher than
500 if non-immediate requests that were issued on other connections
in the same session advance the CmdSN.

If the Login Request is a leading Login Request, the target MUST use
the value presented in the CmdSN as the target value for the
ExpCmdSN.

11.12.9. ExpStatSN

For the first Login Request on a connection, this is the ExpStatSN
for the old connection, and this field is only valid if the Login
Request restarts a connection (see Section 6.3.4).

For subsequent Login Requests, it is used to acknowledge the Login
Responses with their increasing StatSN values.

11.12.10. Login Parameters

The initiator MUST provide some basic parameters in order to enable
the target to determine if the initiator may use the target's
resources and the initial text parameters for the security exchange.

All the rules specified in Section 11.10.5 for Text Requests also
hold for Login Requests. Keys and their explanations are listed in
Section 12 (security negotiation keys) and in Section 13 (operational
parameter negotiation keys). All keys listed in Section 13, except
for the X extension formats, MUST be supported by iSCSI initiators
and targets. Keys listed in Section 12 only need to be supported
when the function to which they refer is mandatory to implement.

11.13. Login Response

The Login Response indicates the progress and/or end of the Login
Phase.

11.13.1. Version-max

All Login Responses within the Login Phase MUST carry the same
Version-max.

The initiator MUST use the value presented as a response to the first
Login Request.

11.13.2. Version-active

Version-active indicates the highest version supported by the target
and initiator. If the target does not support a version within the
range specified by the initiator, the target rejects the login and
this field indicates the lowest version supported by the target.

All Login Responses within the Login Phase MUST carry the same
Version-active.

The initiator MUST use the value presented as a response to the first
Login Request.

11.13.3. TSIH

The TSIH is the target-assigned session-identifying handle. Its
internal format and content are not defined by this protocol, except
for the value 0, which is reserved. With the exception of the Login
Final-Response in a new session, this field should be set to the TSIH
provided by the initiator in the Login Request. For a new session,
the target MUST generate a non-zero TSIH and ONLY return it in the
Login Final-Response (see Section 6.3).

11.13.4. StatSN

For the first Login Response (the response to the first Login
Request), this is the starting status sequence number for the
connection. The next response of any kind -- including the next
Login Response, if any, in the same Login Phase -- will carry this
number + 1. This field is only valid if the Status-Class is 0.

11.13.5. Status-Class and Status-Detail

The Status returned in a Login Response indicates the execution
status of the Login Phase. The status includes:

Status-Class
Status-Detail

A Status-Class of 0 indicates success.

A non-zero Status-Class indicates an exception. In this case,
Status-Class is sufficient for a simple initiator to use when
handling exceptions, without having to look at the Status-Detail.

The Status-Detail allows finer-grained exception handling for more
sophisticated initiators and for better information for logging.

The Status-Classes are as follows:

0 Success - indicates that the iSCSI target successfully
received, understood, and accepted the request. The numbering
fields (StatSN, ExpCmdSN, MaxCmdSN) are only valid if Status-
Class is 0.
1 Redirection - indicates that the initiator must take further
action to complete the request. This is usually due to the
target moving to a different address. All of the redirection
Status-Class responses MUST return one or more text key
parameters of the type "TargetAddress", which indicates the
target's new address. A redirection response MAY be issued by
a target prior to or after completing a security negotiation if
a security negotiation is required. A redirection SHOULD be
accepted by an initiator, even without having the target
complete a security negotiation if any security negotiation is
required, and MUST be accepted by the initiator after the
completion of the security negotiation if any security
negotiation is required.
2 Initiator Error (not a format error) - indicates that the
initiator most likely caused the error. This MAY be due to a
request for a resource for which the initiator does not have
permission. The request should not be tried again.
3 Target Error - indicates that the target sees no errors in the
initiator's Login Request but is currently incapable of
fulfilling the request. The initiator may retry the same Login
Request later.

The table below shows all of the currently allocated status codes.
The codes are in hexadecimal; the first byte is the Status-Class, and
the second byte is the status detail.

matching response, and the NSG is set to FullFeaturePhase in
both the request and the matching response, the Login Phase is
finished, and the initiator may proceed to issue SCSI commands.

If the Status-Class is not 0, the initiator and target MUST close the
TCP connection.

If the target wishes to reject the Login Request for more than one
reason, it should return the primary reason for the rejection.

11.13.6. T (Transit) Bit

The T bit is set to 1 as an indicator of the end of the stage. If
the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
this is also the Login Final-Response (see Section 6.3). A T bit of
0 indicates a "partial" response, which means "more negotiation
needed".

A Login Response with the T bit set to 1 MUST NOT contain key=value
pairs that may require additional answers from the initiator within
the same stage.

If the Status-Class is 0, the T bit MUST NOT be set to 1 if the T bit
in the request was set to 0.

11.13.7. C (Continue) Bit

When set to 1, this bit indicates that the text (set of key=value
pairs) in this Login Response is not complete (it will be continued
on subsequent Login Responses); otherwise, it indicates that this
Login Response ends a set of key=value pairs. A Login Response with
the C bit set to 1 MUST have the T bit set to 0.

11.13.8. Login Parameters

The target MUST provide some basic parameters in order to enable the
initiator to determine if it is connected to the correct port and the
initial text parameters for the security exchange.

All the rules specified in Section 11.11.6 for Text Responses also
hold for Login Responses. Keys and their explanations are listed in
Section 12 (security negotiation keys) and in Section 13 (operational
parameter negotiation keys). All keys listed in Section 13, except
for the X extension formats, MUST be supported by iSCSI initiators
and targets. Keys listed in Section 12 only need to be supported
when the function to which they refer is mandatory to implement.

11.14. Logout Request

The Logout Request is used to perform a controlled closing of a
connection.

An initiator MAY use a Logout Request to remove a connection from a
session or to close an entire session.

After sending the Logout Request PDU, an initiator MUST NOT send any
new iSCSI requests on the closing connection. If the Logout Request
is intended to close the session, new iSCSI requests MUST NOT be sent
on any of the connections participating in the session.

When receiving a Logout Request with the reason code "close the
connection" or "close the session", the target MUST terminate all
pending commands, whether acknowledged via the ExpCmdSN or not, on
that connection or session, respectively.

When receiving a Logout Request with the reason code "remove the
connection for recovery", the target MUST discard all requests not
yet acknowledged via the ExpCmdSN that were issued on the specified
connection and suspend all data/status/R2T transfers on behalf of
pending commands on the specified connection.

The target then issues the Logout Response and half-closes the TCP
connection (sends FIN). After receiving the Logout Response and
attempting to receive the FIN (if still possible), the initiator MUST
completely close the logging-out connection. For the terminated
commands, no additional responses should be expected.

A Logout for a CID may be performed on a different transport
connection when the TCP connection for the CID has already been
terminated. In such a case, only a logical "closing" of the iSCSI
connection for the CID is implied with a Logout.

All commands that were not terminated or not completed (with status)
and acknowledged when the connection is closed completely can be
reassigned to a new connection if the target supports connection
recovery.

If an initiator intends to start recovery for a failing connection,
it MUST use the Logout Request to "clean up" the target end of a
failing connection and enable recovery to start, or use the Login
Request with a non-zero TSIH and the same CID on a new connection for
the same effect. In sessions with a single connection, the
connection can be closed and then a new connection reopened. A
connection reinstatement login can be used for recovery (see
Section 6.3.4).

A successful completion of a Logout Request with the reason code
"close the connection" or "remove the connection for recovery"
results at the target in the discarding of unacknowledged commands
received on the connection being logged out. These are commands that
have arrived on the connection being logged out but that have not
been delivered to SCSI because one or more commands with a smaller
CmdSN have not been received by iSCSI. See Section 4.2.2.1. The
resulting holes in the command sequence numbers will have to be
handled by appropriate recovery (see Section 7), unless the session
is also closed.

The entire logout discussion in this section is also applicable for
an implicit Logout realized by way of a connection reinstatement or
session reinstatement. When a Login Request performs an implicit
Logout, the implicit Logout is performed as if having the reason
codes specified below:

closed, and all commands associated with it, if any, are
to be prepared for a new allegiance.

All other values are reserved.

11.14.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.14.3. CID

This is the connection ID of the connection to be closed (including
closing the TCP stream). This field is only valid if the reason code
is not "close the session".

11.14.4. ExpStatSN

This is the last ExpStatSN value for the connection to be closed.

11.14.5. Implicit Termination of Tasks

A target implicitly terminates the active tasks due to the iSCSI
protocol in the following cases:

a) When a connection is implicitly or explicitly logged out with

the reason code "close the connection" and there are active
tasks allegiant to that connection.

b) When a connection fails and eventually the connection state

times out (state transition M1 in Section 8.2.2) and there are
active tasks allegiant to that connection.

c) When a successful recovery Logout is performed while there are

active tasks allegiant to that connection and those tasks
eventually time out after the Time2Wait and Time2Retain periods
without allegiance reassignment.

d) When a connection is implicitly or explicitly logged out with

the reason code "close the session" and there are active tasks
in that session.

If the tasks terminated in any of the above cases are SCSI tasks,
they must be internally terminated as if with CHECK CONDITION status.
This status is only meaningful for appropriately handling the
internal SCSI state and SCSI side effects with respect to ordering,
because this status is never communicated back as a terminating
status to the initiator. However, additional actions may have to be
taken at the SCSI level, depending on the SCSI context as defined by
the SCSI standards (e.g., queued commands and ACA; UA for the next
command on the I_T nexus in cases a), b), and c) above). After the
tasks are terminated, the target MUST report a Unit Attention
condition on the next command processed on any connection for each
affected I_T_L nexus with the status of CHECK CONDITION, the ASC/ASCQ
value of 47h/7Fh ("SOME COMMANDS CLEARED BY ISCSI PROTOCOL EVENT"),
etc.; see [SPC3].

11.15. Logout Response

The Logout Response is used by the target to indicate if the cleanup
operation for the connection(s) has completed.

After Logout, the TCP connection referred by the CID MUST be closed
at both ends (or all connections must be closed if the logout reason
was session close).

11.15.1. Response

2 - connection recovery is not supported (i.e., the Logout reason
code was "remove the connection for recovery" and the target
does not support it as indicated by the operational
ErrorRecoveryLevel).

- cleanup failed for various reasons.

11.15.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.15.3. Time2Wait

If the Logout response code is 0 and the operational
ErrorRecoveryLevel is 2, this is the minimum amount of time, in
seconds, to wait before attempting task reassignment. If the Logout
response code is 0 and the operational ErrorRecoveryLevel is less
than 2, this field is to be ignored.

This field is invalid if the Logout response code is 1.

If the Logout response code is 2 or 3, this field specifies the
minimum time to wait before attempting a new implicit or explicit
logout.

If Time2Wait is 0, the reassignment or a new Logout may be attempted
immediately.

11.15.4. Time2Retain

If the Logout response code is 0 and the operational
ErrorRecoveryLevel is 2, this is the maximum amount of time, in
seconds, after the initial wait (Time2Wait) that the target waits for
the allegiance reassignment for any active task, after which the task
state is discarded. If the Logout response code is 0 and the
operational ErrorRecoveryLevel is less than 2, this field is to be
ignored.

This field is invalid if the Logout response code is 1.

If the Logout response code is 2 or 3, this field specifies the
maximum amount of time, in seconds, after the initial wait
(Time2Wait) that the target waits for a new implicit or explicit
logout.

If it is the last connection of a session, the whole session state is
discarded after Time2Retain.

If Time2Retain is 0, the target has already discarded the connection
(and possibly the session) state along with the task states. No
reassignment or Logout is required in this case.

If the implementation supports ErrorRecoveryLevel greater than zero,
it MUST support all SNACK types.

The SNACK is used by the initiator to request the retransmission of
numbered responses, data, or R2T PDUs from the target. The SNACK
Request indicates the numbered responses or data "runs" whose
retransmission is requested, where the run starts with the first
StatSN, DataSN, or R2TSN whose retransmission is requested and
indicates the number of Status, Data, or R2T PDUs requested,
including the first. 0 has special meaning when used as a starting
number and length:

When used in RunLength, it means all PDUs starting with the
initial.

When used in both BegRun and RunLength, it means all
unacknowledged PDUs.

The numbered response(s) or R2T(s) requested by a SNACK MUST be
delivered as exact replicas of the ones that the target transmitted
originally, except for the fields ExpCmdSN, MaxCmdSN, and ExpDataSN,
which MUST carry the current values. R2T(s)requested by SNACK MUST
also carry the current value of the StatSN.

The numbered Data-In PDUs requested by a Data SNACK MUST be delivered
as exact replicas of the ones that the target transmitted originally,
except for the fields ExpCmdSN and MaxCmdSN, which MUST carry the
current values; and except for resegmentation (see Section 11.16.3).

Any SNACK that requests a numbered response, data, or R2T that was
not sent by the target or was already acknowledged by the initiator
MUST be rejected with a reason code of "Protocol Error".

11.16.1. Type

This field encodes the SNACK function as follows:

0 - Data/R2T SNACK: requesting retransmission of one or more

Data-In or R2T PDUs.

1 - Status SNACK: requesting retransmission of one or more

numbered responses.

- DataACK: positively acknowledges Data-In PDUs.

3 - R-Data SNACK: requesting retransmission of Data-In PDUs with

possible resegmentation and status tagging.

All other values are reserved.

Data/R2T SNACK, Status SNACK, or R-Data SNACK for a command MUST
precede status acknowledgment for the given command.

11.16.2. Data Acknowledgment

If an initiator operates at ErrorRecoveryLevel 1 or higher, it MUST
issue a SNACK of type DataACK after receiving a Data-In PDU with the
A bit set to 1. However, if the initiator has detected holes in the
input sequence, it MUST postpone issuing the SNACK of type DataACK
until the holes are filled. An initiator MAY ignore the A bit if it
deems that the bit is being set aggressively by the target (i.e.,
before the MaxBurstLength limit is reached).

The DataACK is used to free resources at the target and not to
request or imply data retransmission.

An initiator MUST NOT request retransmission for any data it had
already acknowledged.

11.16.3. Resegmentation

If the initiator MaxRecvDataSegmentLength changed between the
original transmission and the time the initiator requests
retransmission, the initiator MUST issue a R-Data SNACK (see
Section 11.16.1). With R-Data SNACK, the initiator indicates that it
discards all the unacknowledged data and expects the target to resend
it. It also expects resegmentation. In this case, the retransmitted
Data-In PDUs MAY be different from the ones originally sent in order
to reflect changes in MaxRecvDataSegmentLength. Their DataSN starts
with the BegRun of the last DataACK received by the target if any was
received; otherwise, it starts with 0 and is increased by 1 for each
resent Data-In PDU.

A target that has received a R-Data SNACK MUST return a SCSI Response
that contains a copy of the SNACK Tag field from the R-Data SNACK in
the SCSI Response SNACK Tag field as its last or only Response. For
example, if it has already sent a response containing another value
in the SNACK Tag field or had the status included in the last Data-In
PDU, it must send a new SCSI Response PDU. If a target sends more
than one SCSI Response PDU due to this rule, all SCSI Response PDUs
must carry the same StatSN (see Section 11.4.4). If an initiator
attempts to recover a lost SCSI Response (with a Status-SNACK; see
Section 11.16.1) when more than one response has been sent, the
target will send the SCSI Response with the latest content known to
the target, including the last SNACK Tag for the command.

For considerations in allegiance reassignment of a task to a
connection with a different MaxRecvDataSegmentLength, refer to
Section 7.2.2.

11.16.4. Initiator Task Tag

For a Status SNACK and DataACK, the Initiator Task Tag MUST be set to
the reserved value 0xffffffff. In all other cases, the Initiator
Task Tag field MUST be set to the Initiator Task Tag of the
referenced command.

11.16.5. Target Transfer Tag or SNACK Tag

For a R-Data SNACK, this field MUST contain a value that is different
from 0 or 0xffffffff and is unique for the task (identified by the
Initiator Task Tag). This value MUST be copied by the iSCSI target
in the last or only SCSI Response PDU it issues for the command.

For DataACK, the Target Transfer Tag MUST contain a copy of the
Target Transfer Tag and LUN provided with the SCSI Data-In PDU with
the A bit set to 1.

In all other cases, the Target Transfer Tag field MUST be set to the
reserved value 0xffffffff.

11.16.6. BegRun

This field indicates the DataSN, R2TSN, or StatSN of the first PDU
whose retransmission is requested (Data/R2T and Status SNACK), or the
next expected DataSN (DataACK SNACK).

A BegRun of 0, when used in conjunction with a RunLength of 0, means
"resend all unacknowledged Data-In, R2T or Response PDUs".

BegRun MUST be 0 for a R-Data SNACK.

11.16.7. RunLength

This field indicates the number of PDUs whose retransmission is
requested.

A RunLength of 0 signals that all Data-In, R2T, or Response PDUs
carrying the numbers equal to or greater than BegRun have to be
resent.

The RunLength MUST also be 0 for a DataACK SNACK in addition to a
R-Data SNACK.

For iSCSI, Data-Out PDU retransmission is only done if the
target requests retransmission with a recovery R2T. However,
if this is the data digest error on immediate data, the
initiator may choose to retransmit the whole PDU, including
the immediate data.

Note 2:

A target should use this reason code for all invalid values
of PDU fields that are meant to describe a task, a response,
or a data transfer. Some examples are invalid TTT/ITT,
buffer offset, LUN qualifying a TTT, and an invalid sequence
number in a SNACK.

Note 3:

Reason code 0x0b ("Negotiation Reset") as defined in
Section 10.17.1 of [RFC3720] is deprecated and MUST NOT be
used by implementations. An implementation receiving reason
code 0x0b MUST treat it as a negotiation failure that
terminates the Login Phase and the TCP connection, as
specified in Section 7.12.

All other values for Reason are unassigned.

In all the cases in which a pre-instantiated SCSI task is terminated
because of the reject, the target MUST issue a proper SCSI command
response with CHECK CONDITION as described in Section 11.4.3. In
these cases in which a status for the SCSI task was already sent
before the reject, no additional status is required. If the error is
detected while data from the initiator is still expected (i.e., the
command PDU did not contain all the data and the target has not
received a Data-Out PDU with the Final bit set to 1 for the
unsolicited data, if any, and all outstanding R2Ts, if any), the
target MUST wait until it receives the last expected Data-Out PDUs
with the F bit set to 1 before sending the Response PDU.

For additional usage semantics of the Reject PDU, see Section 7.3.

11.17.2. DataSN/R2TSN

This field is only valid if the rejected PDU is a Data/R2T SNACK and
the Reject reason code is "Protocol Error" (see Section 11.16). The
DataSN/R2TSN is the next Data/R2T sequence number that the target
would send for the task, if any.

11.17.3. StatSN, ExpCmdSN, and MaxCmdSN

These fields carry their usual values and are not related to the
rejected command. The StatSN is advanced after a Reject.

11.17.4. Complete Header of Bad PDU

The target returns the header (not including the digest) of the PDU
in error as the data of the response.

NOP-Out may be used by an initiator as a "ping request" to verify
that a connection/session is still active and all its components are
operational. The NOP-In response is the "ping echo".

A NOP-Out is also sent by an initiator in response to a NOP-In.

A NOP-Out may also be used to confirm a changed ExpStatSN if another
PDU will not be available for a long time.

Upon receipt of a NOP-In with the Target Transfer Tag set to a valid
value (not the reserved value 0xffffffff), the initiator MUST respond
with a NOP-Out. In this case, the NOP-Out Target Transfer Tag MUST
contain a copy of the NOP-In Target Transfer Tag. The initiator

SHOULD NOT send a NOP-Out in response to any other received NOP-In,
in order to avoid lengthy sequences of NOP-In and NOP-Out PDUs sent
in response to each other.

11.18.1. Initiator Task Tag

The NOP-Out MUST have the Initiator Task Tag set to a valid value
only if a response in the form of a NOP-In is requested (i.e., the
NOP-Out is used as a ping request). Otherwise, the Initiator Task
Tag MUST be set to 0xffffffff.

When a target receives the NOP-Out with a valid Initiator Task Tag,
it MUST respond with a NOP-In Response (see Section 4.6.3.6).

If the Initiator Task Tag contains 0xffffffff, the I bit MUST be set
to 1, and the CmdSN is not advanced after this PDU is sent.

11.18.2. Target Transfer Tag

The Target Transfer Tag is a target-assigned identifier for the
operation.

The NOP-Out MUST only have the Target Transfer Tag set if it is
issued in response to a NOP-In with a valid Target Transfer Tag. In
this case, it copies the Target Transfer Tag from the NOP-In PDU.
Otherwise, the Target Transfer Tag MUST be set to 0xffffffff.

When the Target Transfer Tag is set to a value other than 0xffffffff,
the LUN field MUST also be copied from the NOP-In.

11.18.3. Ping Data

Ping data is reflected in the NOP-In Response. The length of the
reflected data is limited to MaxRecvDataSegmentLength. The length of
ping data is indicated by the DataSegmentLength. 0 is a valid value
for the DataSegmentLength and indicates the absence of ping data.

NOP-In is sent by a target as either a response to a NOP-Out, a
"ping" to an initiator, or a means to carry a changed ExpCmdSN and/or
MaxCmdSN if another PDU will not be available for a long time (as
determined by the target).

When a target receives the NOP-Out with a valid Initiator Task Tag
(not the reserved value 0xffffffff), it MUST respond with a NOP-In
with the same Initiator Task Tag that was provided in the NOP-Out
request. It MUST also duplicate up to the first
MaxRecvDataSegmentLength bytes of the initiator-provided Ping Data.
For such a response, the Target Transfer Tag MUST be 0xffffffff. The
target SHOULD NOT send a NOP-In in response to any other received
NOP-Out in order to avoid lengthy sequences of NOP-In and NOP-Out
PDUs sent in response to each other.

Otherwise, when a target sends a NOP-In that is not a response to a
NOP-Out received from the initiator, the Initiator Task Tag MUST be
set to 0xffffffff, and the data segment MUST NOT contain any data
(DataSegmentLength MUST be 0).

11.19.1. Target Transfer Tag

If the target is responding to a NOP-Out, this field is set to the
reserved value 0xffffffff.

If the target is sending a NOP-In as a ping (intending to receive a
corresponding NOP-Out), this field is set to a valid value (not the
reserved value 0xffffffff).

If the target is initiating a NOP-In without wanting to receive a
corresponding NOP-Out, this field MUST hold the reserved value
0xffffffff.

11.19.2. StatSN

The StatSN field will always contain the next StatSN. However, when
the Initiator Task Tag is set to 0xffffffff, the StatSN for the
connection is not advanced after this PDU is sent.

11.19.3. LUN

A LUN MUST be set to a correct value when the Target Transfer Tag is
valid (not the reserved value 0xffffffff).

12. iSCSI Security Text Keys and Authentication Methods

Only the following keys are used during the SecurityNegotiation stage
of the Login Phase:

The AuthMethod selection is followed by an "authentication exchange"
specific to the authentication method selected.

The authentication method proposal may be made by either the
initiator or the target. However, the initiator MUST make the first
step specific to the selected authentication method as soon as it is
selected. It follows that if the target makes the authentication
method proposal, the initiator sends the first key(s) of the exchange
together with its authentication method selection.

The authentication exchange authenticates the initiator to the target
and, optionally, the target to the initiator. Authentication is
OPTIONAL to use but MUST be supported by the target and initiator.

The initiator and target MUST implement CHAP. All other
authentication methods are OPTIONAL.

Private or public extension algorithms MAY also be negotiated for
authentication methods. Whenever a private or public extension
algorithm is part of the default offer (the offer made in the absence
of explicit administrative action), the implementer MUST ensure that
CHAP is listed as an alternative in the default offer and "None" is
not part of the default offer.

Extension authentication methods MUST be named using one of the
following two formats:

1) Z-reversed.vendor.dns_name.do_something=

New public key with no name prefix constraints

Authentication methods named using the Z- format are used as private
extensions. New public keys must be registered with IANA using the
IETF Review process ([RFC5226]). New public extensions for
authentication methods MUST NOT use the Z# name prefix.

For all of the public or private extension authentication methods,
the method-specific keys MUST conform to the format specified in
Section 6.1 for standard-label.

To identify the vendor for private extension authentication methods,
we suggest using the reversed DNS-name as a prefix to the proper
digest names.

The part of digest-name following Z- MUST conform to the format for
standard-label specified in Section 6.1.

Support for public or private extension authentication methods is
OPTIONAL.

The following subsections define the specific exchanges for each of
the standardized authentication methods. As mentioned earlier, the
first step is always done by the initiator.

12.1.1. Kerberos

The default principal name assumed by an iSCSI initiator or target
(prior to any administrative configuration action) MUST be the iSCSI
Initiator Name or iSCSI Target Name, respectively, prefixed by the
string "iscsi/".

If the initiator authentication fails, the target MUST respond with a
Login reject with "Authentication Failure" status. Otherwise, if the
initiator has selected the mutual authentication option (by setting
MUTUAL-REQUIRED in the ap-options field of the KRB_AP_REQ), the
target MUST reply with:

KRB_AP_REP=<KRB_AP_REP>

where KRB_AP_REP is the server's response message as defined in
[RFC4120].

If mutual authentication was selected and target authentication
fails, the initiator MUST close the connection.

KRB_AP_REQ and KRB_AP_REP are binary-values, and their binary length
(not the length of the character string that represents them in
encoded form) MUST NOT exceed 65536 bytes. Hex or Base64 encoding
may be used for KRB_AP_REQ and KRB_AP_REP; see Section 6.1.

12.1.2. Secure Remote Password (SRP)

The target MUST answer with a Login reject with the "Authorization
Failure" status or reply with:

SRP_GROUP=<G1,G2...> SRP_s=<s>

where G1,G2... are proposed groups, in order of preference.

The initiator MUST either close the connection or continue with:

SRP_A=<A> SRP_GROUP=<G>

where G is one of G1,G2... that were proposed by the target.

The target MUST answer with a Login reject with the "Authentication
Failure" status or reply with:

SRP_B=<B>

The initiator MUST close the connection or continue with:

SRP_M=<M>

If the initiator authentication fails, the target MUST answer with a
Login reject with "Authentication Failure" status. Otherwise, if the
initiator sent TargetAuth=Yes in the first message (requiring target
authentication), the target MUST reply with:

SRP_HM=<H(A | M | K)>

If the target authentication fails, the initiator MUST close the
connection:

where U, s, A, B, M, and H(A | M | K) are defined in [RFC2945] (using
the SHA1 hash function, such as SRP-SHA1)
and
G,Gn ("Gn" stands for G1,G2...) are identifiers of SRP groups
specified in [RFC3723].

G, Gn, and U are text strings; s,A,B,M, and H(A | M | K) are
binary-values. The length of s,A,B,M and H(A | M | K) in binary form
(not the length of the character string that represents them in
encoded form) MUST NOT exceed 1024 bytes. Hex or Base64 encoding may
be used for s,A,B,M and H(A | M | K); see Section 6.1.

See Appendix B for the related login example.

For the SRP_GROUP, all the groups specified in [RFC3723] up to
1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) must be
supported by initiators and targets. To guarantee interoperability,
targets MUST always offer "SRP-1536" as one of the proposed groups.

12.1.3. Challenge Handshake Authentication Protocol (CHAP)

The target MUST answer with a Login reject with the "Authentication
Failure" status or reply with:

CHAP_A=<A> CHAP_I=<I> CHAP_C=<C>

where A is one of A1,A2... that were proposed by the initiator.

The initiator MUST continue with:

CHAP_N=<N> CHAP_R=<R>

or, if it requires target authentication, with:

CHAP_N=<N> CHAP_R=<R> CHAP_I=<I> CHAP_C=<C>

If the initiator authentication fails, the target MUST answer with a
Login reject with "Authentication Failure" status. Otherwise, if the
initiator required target authentication, the target MUST either
answer with a Login reject with "Authentication Failure" or reply
with:

CHAP_N=<N> CHAP_R=<R>

If the target authentication fails, the initiator MUST close the
connection:

where N, (A,A1,A2), I, C, and R are (correspondingly) the Name,
Algorithm, Identifier, Challenge, and Response as defined in
[RFC1994].

N is a text string; A,A1,A2, and I are numbers; C and R are
binary-values. Their binary length (not the length of the character
string that represents them in encoded form) MUST NOT exceed
1024 bytes. Hex or Base64 encoding may be used for C and R; see
Section 6.1.

See Appendix B for the related login example.

For the Algorithm, as stated in [RFC1994], one value is required to
be implemented:

5 (CHAP with MD5)

To guarantee interoperability, initiators MUST always offer it as one
of the proposed algorithms.

13. Login/Text Operational Text Keys

Some session-specific parameters MUST only be carried on the leading
connection and cannot be changed after the leading connection login
(e.g., MaxConnections -- the maximum number of connections). This
holds for a single connection session with regard to connection
restart. The keys that fall into this category have the "use: LO"
(Leading Only).

Keys that can only be used during login have the "use: IO"
(Initialize Only), while those that can be used in both the Login
Phase and Full Feature Phase have the "use: ALL".

Keys that can only be used during the Full Feature Phase use FFPO
(Full Feature Phase Only).

Keys marked as Any-Stage may also appear in the SecurityNegotiation
stage, while all other keys described in this section are
operational keys.

Keys that do not require an answer are marked as Declarative.

Key scope is indicated as session-wide (SW) or connection-only (CO).

"Result function", wherever mentioned, states the function that can
be applied to check the validity of the responder selection.
"Minimum" means that the selected value cannot exceed the offered
value. "Maximum" means that the selected value cannot be lower than
the offered value. "AND" means that the selected value must be a
possible result of a Boolean "and" function with an arbitrary Boolean
value (e.g., if the offered value is No the selected value must be
No). "OR" means that the selected value must be a possible result of
a Boolean "or" function with an arbitrary Boolean value (e.g., if the
offered value is Yes the selected value must be Yes).

13.1. HeaderDigest and DataDigest

Digests enable the checking of end-to-end, non-cryptographic data
integrity beyond the integrity checks provided by the link layers and
the covering of the whole communication path, including all elements
that may change the network-level PDUs, such as routers, switches,
and proxies.

The following table lists cyclic integrity checksums that can be
negotiated for the digests and MUST be implemented by every iSCSI
initiator and target. These digest options only have error detection
significance.

When the initiator and target agree on a digest, this digest MUST be
used for every PDU in the Full Feature Phase.

Padding bytes, when present in a segment covered by a CRC, SHOULD be
set to 0 and are included in the CRC.

The CRC MUST be calculated by a method that produces the same results
as the following process:

The PDU bits are considered as the coefficients of a polynomial
M(x) of degree n - 1; bit 7 of the lowest numbered byte is
considered the most significant bit (x**n - 1), followed by bit 6
of the lowest numbered byte through bit 0 of the highest numbered
byte (x**0).

The most significant 32 bits are complemented.

- The polynomial is multiplied by x**32, then divided by G(x). The
generator polynomial produces a remainder R(x) of degree <= 31.

The coefficients of R(x) are formed into a 32-bit sequence.

The bit sequence is complemented, and the result is the CRC.

The CRC bits are mapped into the digest word. The x**31
coefficient is mapped to bit 7 of the lowest numbered byte of the
digest, and the mapping continues with successive coefficients and
bits so that the x**24 coefficient is mapped to bit 0 of the lowest
numbered byte. The mapping continues further with the x**23
coefficient mapped to bit 7 of the next byte in the digest until
the x**0 coefficient is mapped to bit 0 of the highest numbered
byte of the digest.

Computing the CRC over any segment (data or header) extended to
include the CRC built using the generator 0x11edc6f41 will always
get the value 0x1c2d19ed as its final remainder (R(x)). This value
is given here in its polynomial form (i.e., not mapped as the
digest word).

For a discussion about selection criteria for the CRC, see [RFC3385].
For a detailed analysis of the iSCSI polynomial, see [Castagnoli93].

Private or public extension algorithms MAY also be negotiated for
digests. Whenever a private or public digest extension algorithm is
part of the default offer (the offer made in the absence of explicit
administrative action), the implementer MUST ensure that CRC32C is
listed as an alternative in the default offer and "None" is not part
of the default offer.

Extension digest algorithms MUST be named using one of the following
two formats:

1) Y-reversed.vendor.dns_name.do_something=

New public key with no name prefix constraints

Digests named using the Y- format are used for private purposes
(unregistered). New public keys must be registered with IANA using
the IETF Review process ([RFC5226]). New public extensions for
digests MUST NOT use the Y# name prefix.

For private extension digests, to identify the vendor we suggest
using the reversed DNS-name as a prefix to the proper digest names.

The part of digest-name following Y- MUST conform to the format for
standard-label specified in Section 6.1.

The initiator of the TCP connection MUST provide this key to the
remote endpoint in the first Login Request if the initiator is not
establishing a Discovery session. The iSCSI Target Name specifies
the worldwide unique name of the target.

The TargetName key may also be returned by the SendTargets Text
Request (which is its only use when issued by a target).

The initiator of the TCP connection MUST provide this key to the
remote endpoint at the first login of the Login Phase for every
connection. The InitiatorName key enables the initiator to identify
itself to the remote endpoint.

The InitiatorName MUST NOT be redeclared within the Login Phase.

13.6. TargetAlias

If a target has been configured with a human-readable name or
description, this name SHOULD be communicated to the initiator during
a Login Response PDU if SessionType=Normal (see Section 13.21). This
string is not used as an identifier, nor is it meant to be used for
authentication or authorization decisions. It can be displayed by
the initiator's user interface in a list of targets to which it is
connected.

13.7. InitiatorAlias

If an initiator has been configured with a human-readable name or
description, it SHOULD be communicated to the target during a Login
Request PDU. If not, the host name can be used instead. This string
is not used as an identifier, nor is it meant to be used for
authentication or authorization decisions. It can be displayed by
the target's user interface in a list of initiators to which it is
connected.

13.9. TargetPortalGroupTag

The TargetPortalGroupTag key is a 16-bit binary-value that uniquely
identifies a portal group within an iSCSI target node. This key
carries the value of the tag of the portal group that is servicing
the Login Request. The iSCSI target returns this key to the
initiator in the Login Response PDU to the first Login Request PDU
that has the C bit set to 0 when TargetName is given by the
initiator.

[SAM2] notes in its informative text that the TPGT value should be
non-zero; note that this is incorrect. A zero value is allowed as a
legal value for the TPGT. This discrepancy currently stands
corrected in [SAM4].

For the complete usage expectations of this key, see Section 6.3.

13.10. InitialR2T

The InitialR2T key is used to turn off the default use of R2T for
unidirectional operations and the output part of bidirectional
commands, thus allowing an initiator to start sending data to a
target as if it has received an initial R2T with Buffer
Offset=Immediate Data Length and Desired Data Transfer
Length=(min(FirstBurstLength, Expected Data Transfer Length) -
Received Immediate Data Length).

The default action is that R2T is required, unless both the initiator
and the target send this key-pair attribute specifying InitialR2T=No.
Only the first outgoing data burst (immediate data and/or separate
PDUs) can be sent unsolicited (i.e., not requiring an explicit R2T).

13.11. ImmediateData

The initiator and target negotiate support for immediate data. To
turn immediate data off, the initiator or target must state its
desire to do so. ImmediateData can be turned on if both the
initiator and target have ImmediateData=Yes.

If ImmediateData is set to Yes and InitialR2T is set to Yes
(default), then only immediate data are accepted in the first burst.

If ImmediateData is set to No and InitialR2T is set to Yes, then the
initiator MUST NOT send unsolicited data and the target MUST reject
unsolicited data with the corresponding response code.

If ImmediateData is set to No and InitialR2T is set to No, then the
initiator MUST NOT send unsolicited immediate data but MAY send one
unsolicited burst of Data-OUT PDUs.

If ImmediateData is set to Yes and InitialR2T is set to No, then the
initiator MAY send unsolicited immediate data and/or one unsolicited
burst of Data-OUT PDUs.

13.13. MaxBurstLength

The initiator and target negotiate the maximum SCSI data payload in
bytes in a Data-In or a solicited Data-Out iSCSI sequence. A
sequence consists of one or more consecutive Data-In or Data-Out PDUs
that end with a Data-In or Data-Out PDU with the F bit set to 1.

The initiator and target negotiate the maximum amount in bytes of
unsolicited data an iSCSI initiator may send to the target during the
execution of a single SCSI command. This covers the immediate data
(if any) and the sequence of unsolicited Data-Out PDUs (if any) that
follow the command.

FirstBurstLength MUST NOT exceed MaxBurstLength.

13.15. DefaultTime2Wait

Use: LO
Senders: Initiator and target
Scope: SW

DefaultTime2Wait=<numerical-value-0-to-3600>

Default is 2.
Result function is Maximum.

The initiator and target negotiate the minimum time, in seconds, to
wait before attempting an explicit/implicit logout or an active task
reassignment after an unexpected connection termination or a
connection reset.

A value of 0 indicates that logout or active task reassignment can be
attempted immediately.

13.16. DefaultTime2Retain

Use: LO
Senders: Initiator and target
Scope: SW

DefaultTime2Retain=<numerical-value-0-to-3600>

Default is 20.
Result function is Minimum.

The initiator and target negotiate the maximum time, in seconds,
after an initial wait (Time2Wait), before which an active task
reassignment is still possible after an unexpected connection
termination or a connection reset.

This value is also the session state timeout if the connection in
question is the last LOGGED_IN connection in the session.

A value of 0 indicates that connection/task state is immediately
discarded by the target.

13.17. MaxOutstandingR2T

The initiator and target negotiate the maximum number of outstanding
R2Ts per task, excluding any implied initial R2T that might be part
of that task. An R2T is considered outstanding until the last data
PDU (with the F bit set to 1) is transferred or a sequence reception
timeout (Section 7.1.4.1) is encountered for that data sequence.

13.18. DataPDUInOrder

"No" is used by iSCSI to indicate that the data PDUs within sequences
can be in any order. "Yes" is used to indicate that data PDUs within
sequences have to be at continuously increasing addresses and
overlays are forbidden.

13.19. DataSequenceInOrder

A data sequence is a sequence of Data-In or Data-Out PDUs that end
with a Data-In or Data-Out PDU with the F bit set to 1. A Data-Out
sequence is sent either unsolicited or in response to an R2T.
Sequences cover an offset-range.

If DataSequenceInOrder is set to No, data PDU sequences may be
transferred in any order.

If DataSequenceInOrder is set to Yes, data sequences MUST be
transferred using continuously non-decreasing sequence offsets (R2T
buffer offset for writes, or the smallest SCSI Data-In buffer offset
within a read data sequence).

If DataSequenceInOrder is set to Yes, a target may retry at most the
last R2T, and an initiator may at most request retransmission for the
last read data sequence. For this reason, if ErrorRecoveryLevel is
not 0 and DataSequenceInOrder is set to Yes, then MaxOutstandingR2T
MUST be set to 1.

13.20. ErrorRecoveryLevel

Use: LO
Senders: Initiator and target
Scope: SW

ErrorRecoveryLevel=<numerical-value-0-to-2>

Default is 0.
Result function is Minimum.

The initiator and target negotiate the recovery level supported.

Recovery levels represent a combination of recovery capabilities.
Each recovery level includes all the capabilities of the lower
recovery levels and adds some new ones to them.

In the description of recovery mechanisms, certain recovery classes
are specified. Section 7.1.5 describes the mapping between the
classes and the levels.

13.21. SessionType

The initiator indicates the type of session it wants to create. The
target can either accept it or reject it.

A Discovery session indicates to the target that the only purpose of
this session is discovery. The only requests a target accepts in
this type of session are a Text Request with a SendTargets key and a
Logout Request with reason "close the session".

The Discovery session implies MaxConnections = 1 and overrides both
the default and an explicit setting. As Section 7.4.1 states,
ErrorRecoveryLevel MUST be 0 (zero) for Discovery sessions.

Depending on the type of session, a target may decide on resources to
allocate, the security to enforce, etc., for the session. If the
SessionType key is thus going to be offered as "Discovery", it SHOULD
be offered in the initial Login Request by the initiator.

13.22. The Private Extension Key Format

Use: ALL
Senders: Initiator and target
Scope: specific key dependent

X-reversed.vendor.dns_name.do_something=

Keys with this format are used for private extension purposes. These
keys always start with X- if unregistered with IANA (private). New
public keys (if registered with IANA via an IETF Review [RFC5226]) no
longer have an X# name prefix requirement; implementers may propose
any intuitive unique name.

For unregistered keys, to identify the vendor we suggest using the
reversed DNS-name as a prefix to the key-proper.

The part of key-name following X- MUST conform to the format for
key-name specified in Section 6.1.

Vendor-specific keys MUST ONLY be used in Normal sessions.

Support for public or private extension keys is OPTIONAL.

13.23. TaskReporting

This key is used to negotiate the task completion reporting semantics
from the SCSI target. The following table describes the semantics
that an iSCSI target MUST support for respective negotiated key
values. Whenever this key is negotiated, at least the RFC3720 and
ResponseFence values MUST be offered as options by the negotiation
originator.

When TaskReporting is not negotiated to FastAbort, the standard
multi-task abort semantics in Section 4.2.3.3 MUST be used.

13.24. iSCSIProtocolLevel Negotiation

The iSCSIProtocolLevel associated with this document is "1". As a
responder or an originator in a negotiation of this key, an iSCSI
implementation compliant to this document alone, without any future
protocol extensions, MUST use this value as defined by [RFC7144].

13.25. Obsoleted Keys

This document obsoletes the following keys defined in [RFC3720]:
IFMarker, OFMarker, OFMarkInt, and IFMarkInt. However, iSCSI
implementations compliant to this document may still receive these
obsoleted keys -- i.e., in a responder role -- in a text negotiation.

When an IFMarker or OFMarker key is received, a compliant iSCSI
implementation SHOULD respond with the constant "Reject" value. The
implementation MAY alternatively respond with a "No" value.

However, the implementation MUST NOT respond with a "NotUnderstood"
value for either of these keys.

When an IFMarkInt or OFMarkInt key is received, a compliant iSCSI
implementation MUST respond with the constant "Reject" value. The
implementation MUST NOT respond with a "NotUnderstood" value for
either of these keys.

This document does not define the structure or content of the list of
values.

The initiator or target declares the details of its iSCSI node
architecture to the remote endpoint. These details may include, but
are not limited to, iSCSI vendor software, firmware, or hardware
versions; the OS version; or hardware architecture. This key may be
declared on a Discovery session or a Normal session.

The length of the key value (total length of the list-of-values) MUST
NOT be greater than 255 bytes.

X#NodeArchitecture MUST NOT be redeclared during the Login Phase.

13.26.2. Implementation Requirements

Functional behavior of the iSCSI node (this includes the iSCSI
protocol logic -- the SCSI, iSCSI, and TCP/IP protocols) MUST NOT
depend on the presence, absence, or content of the X#NodeArchitecture
key. The key MUST NOT be used by iSCSI nodes for interoperability or
for exclusion of other nodes. To ensure proper use, key values
SHOULD be set by the node itself, and there SHOULD NOT be provisions
for the key values to contain user-defined text.

Nodes implementing this key MUST choose one of the following
implementation options:

only transmit the key,

only log the key values received from other nodes, or

both transmit and log the key values.

Each node choosing to implement transmission of the key values MUST
be prepared to handle the response of iSCSI nodes that do not
understand the key.

Nodes that implement transmission and/or logging of the key values
may also implement administrative mechanisms that disable and/or
change the logging and key transmission details (see Section 9.4).
Thus, a valid behavior for this key may be that a node is completely
silent (the node does not transmit any key value and simply discards
any key values it receives without issuing a NotUnderstood response).

14. Rationale for Revised IANA Considerations

This document makes rather significant changes in this area, and this
section outlines the reasons behind the changes. As previously
specified in [RFC3720], iSCSI had used text string prefixes, such as
X- and X#, to distinguish extended login/text keys, digest
algorithms, and authentication methods from their standardized
counterparts. Based on experience with other protocols, [RFC6648],
however, strongly recommends against this practice, in large part
because extensions that use such prefixes may become standard over
time, at which point it can be infeasible to change their text string
names due to widespread usage under the existing text string name.

iSCSI's experience with public extensions supports the
recommendations in [RFC6648], as the only extension item ever
registered with IANA, the X#NodeArchitecture key, was specified as a
standard key in a Standards Track RFC [RFC4850] and hence did not
require the X# prefix. In addition, that key is the only public
iSCSI extension that has been registered with IANA since RFC 3720 was
originally published, so there has been effectively no use of the X#,
Y#, and Z# public extension formats.

Therefore, this document makes the following changes to the IANA
registration procedures for iSCSI:

The separate registries for X#, Y#, and Z# public extensions
are removed. The single entry in the registry for X#
login/text keys (X#NodeArchitecture) is transferred to the main
"iSCSI Login/Text Keys" registry. IANA has never created the
latter two registries because there have been no registration
requests for them. These public extension formats (X#, Y#, Z#)
MUST NOT be used, with the exception of the existing
X#NodeArchitecture key.

The registration procedures for the main "iSCSI Login/Text
Keys", "iSCSI digests", and "iSCSI authentication methods" IANA
registries are changed to IETF Review [RFC5226] for possible
future extensions to iSCSI. This change includes a deliberate
decision to remove the possibility of specifying an IANA-
registered iSCSI extension in an RFC published via an RFC
Editor Independent Submission, as the level of review in that
process is insufficient for iSCSI extensions.

The restriction against registering items using the private
extension formats (X-, Y-, Z-) in the main IANA registries is
removed. Extensions using these formats MAY be registered
under the IETF Review registration procedures, but each format
is restricted to the type of extension for which it is
specified in this RFC and MUST NOT be used for other types.
For example, the X- extension format for extension login/text
keys MUST NOT be used for digest algorithms or authentication
methods.

15. IANA Considerations

The well-known TCP port number for iSCSI connections assigned by IANA
is 3260, and this is the default iSCSI port. Implementations needing
a system TCP port number may use port 860, the port assigned by IANA
as the iSCSI system port; however, in order to use port 860, it MUST
be explicitly specified -- implementations MUST NOT default to the
use of port 860, as 3260 is the only allowed default.

IANA has replaced the references for ports 860 and 3260, both TCP and
UDP, with references to this document. Please see
http://www.iana.org/assignments/service-names-port-numbers.

IANA has updated all references to RFC 3720, RFC 4850, and RFC 5048
to instead reference this RFC in all of the iSCSI registries that are
part of the "Internet Small Computer System Interface (iSCSI)
Parameters" set of registries. This change reflects the fact that
those three RFCs are obsoleted by this RFC. References to other RFCs
that are not being obsoleted (e.g., RFC 3723, RFC 5046) should not be
changed.

IANA has performed the following actions on the "iSCSI Login/Text
Keys" registry:

Changed the registration procedure to IETF Review from Standard
Required.

Changed the RFC 5048 reference for the registry to reference
this RFC.

Added the X#NodeArchitecture key from the "iSCSI extended key"
registry, and changed its reference to this RFC.

IANA has changed the registration procedures for the "iSCSI
authentication methods" and "iSCSI digests" registries to IETF Review
from RFC Required.

IANA has removed the "iSCSI extended key" registry, as its one entry
has been added to the "iSCSI Login/Text Keys" registry.

IANA has marked as obsolete the values 4 and 5 for SPKM1 and SPKM2,
respectively, in the "iSCSI authentication methods" subregistry of
the "Internet Small Computer System Interface (iSCSI) Parameters" set
of registries.

IANA has added this document to the "iSCSI Protocol Level" registry
with value 1, as mentioned in Section 13.24.

All the other IANA considerations stated in [RFC3720] and [RFC5048]
remain unchanged. The assignments contained in the following
subregistries are not repeated in this document:

Send Data and Receive Data may be transferred simultaneously as in
an atomic Read-Old-Write-New or sequentially as in an atomic
Read-Update-Write (in the latter case, the R2T may follow the
received data).

If the initiator authentication is successful, the target proceeds
with:

T-> Login (CSG,NSG=0,1 T=1)

I-> Login (CSG,NSG=1,0 T=0)

... iSCSI parameters

T-> Login (CSG,NSG=1,0 T=0)

... iSCSI parameters

And at the end:

I-> Login (CSG,NSG=1,3 T=1)

optional iSCSI parameters
T-> Login (CSG,NSG=1,3 T=1) "login accept"

In the next example, the initiator does not offer any security
parameters. It therefore may offer iSCSI parameters on the Login PDU
with the T bit set to 1, and the target may respond with a final
Login Response PDU immediately:

Appendix C. SendTargets Operation

The text in this appendix is a normative part of this document.

To reduce the amount of configuration required on an initiator, iSCSI
provides the SendTargets Text Request. The initiator uses the
SendTargets request to get a list of targets to which it may have
access, as well as the list of addresses (IP address and TCP port) on
which these targets may be accessed.

To make use of SendTargets, an initiator must first establish one of
two types of sessions. If the initiator establishes the session
using the key "SessionType=Discovery", the session is a Discovery
session, and a target name does not need to be specified. Otherwise,
the session is a Normal operational session. The SendTargets command
MUST only be sent during the Full Feature Phase of a Normal or
Discovery session.

A system that contains targets MUST support Discovery sessions on
each of its iSCSI IP address-port pairs and MUST support the
SendTargets command on the Discovery session. In a Discovery
session, a target MUST return all path information (IP address-port
pairs and Target Portal Group Tags) for the targets on the target
Network Entity that the requesting initiator is authorized to access.

A target MUST support the SendTargets command on operational
sessions; these will only return path information about the target to
which the session is connected and do not need to return information
about other target names that may be defined in the responding
system.

An initiator MAY make use of the SendTargets command as it sees fit.

A SendTargets command consists of a single Text Request PDU. This
PDU contains exactly one text key and value. The text key MUST be
SendTargets. The expected response depends upon the value, as well
as whether the session is a Discovery session or an operational
session.

The value must be one of:

All

The initiator is requesting that information on all relevant
targets known to the implementation be returned. This value
MUST be supported on a Discovery session and MUST NOT be
supported on an operational session.

<iSCSI-target-name>

If an iSCSI Target Name is specified, the session should
respond with addresses for only the named target, if possible.
This value MUST be supported on Discovery sessions. A
Discovery session MUST be capable of returning addresses for
those targets that would have been returned had value=All been
designated.

<nothing>

The session should only respond with addresses for the target
to which the session is logged in. This MUST be supported on
operational sessions and MUST NOT return targets other than the
one to which the session is logged in.

The response to this command is a Text Response that contains a list
of zero or more targets and, optionally, their addresses. Each
target is returned as a target record. A target record begins with
the TargetName text key, followed by a list of TargetAddress text
keys, and bounded by the end of the Text Response or the next
TargetName key, which begins a new record. No text keys other than
TargetName and TargetAddress are permitted within a SendTargets
response.

For the format of the TargetName, see Section 13.4.

A Discovery session MAY respond to a SendTargets request with its
complete list of targets, or with a list of targets that is based on
the name of the initiator logged in to the session.

A SendTargets response MUST NOT contain target names if there are no
targets for the requesting initiator to access.

Each target record returned includes zero or more TargetAddress
fields.

Each target record starts with one text key of the form:

TargetName=<target-name-goes-here>

followed by zero or more address keys of the form:

TargetAddress=<hostname-or-ipaddress>[:<tcp-port>],

<portal-group-tag>

The hostname-or-ipaddress contains a domain name, IPv4 address, or
IPv6 address ([RFC4291]), as specified for the TargetAddress key.

A hostname-or-ipaddress duplicated in TargetAddress responses for a
given node (the port is absent or equal) would probably indicate that
multiple address families are in use at once (IPv6 and IPv4).

Each TargetAddress belongs to a portal group, identified by its
numeric Target Portal Group Tag (see Section 13.9). The iSCSI Target
Name, together with this tag, constitutes the SCSI port identifier;
the tag only needs to be unique within a given target's name list of
addresses.

Multiple-connection sessions can span iSCSI addresses that belong to
the same portal group.

If a SendTargets response reports an iSCSI address for a target, it
SHOULD also report all other addresses in its portal group in the
same response.

A SendTargets Text Response can be longer than a single Text Response
PDU and makes use of the long Text Responses as specified.

After obtaining a list of targets from the Discovery session, an
iSCSI initiator may initiate new sessions to log in to the discovered
targets for full operation. The initiator MAY keep the Discovery
session open and MAY send subsequent SendTargets commands to discover
new targets.

Examples:

This example is the SendTargets response from a single target that
has no other interface ports.

The initiator sends a Text Request that contains:

SendTargets=All

The target sends a Text Response that contains:

TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

All the target had to return in this simple case was the target name.
It is assumed by the initiator that the IP address and TCP port for
this target are the same as those used on the current connection to
the default iSCSI target.

The next example has two internal iSCSI targets, each accessible via
two different ports with different IP addresses. The following is
the Text Response:

Both targets share both addresses; the multiple addresses are likely
used to provide multi-path support. The initiator may connect to
either target name on either address. Each of the addresses has its
own Target Portal Group Tag; they do not support spanning multiple-
connection sessions with each other. Keep in mind that the Target
Portal Group Tags for the two named targets are independent of one
another; portal group "1" on the first target is not necessarily the
same as portal group "1" on the second target.

In the above example, a DNS host name or an IPv6 address could have
been returned instead of an IPv4 address.

The next Text Response shows a target that supports spanning sessions
across multiple addresses and further illustrates the use of the
Target Portal Group Tags:

In this example, any of the target addresses can be used to reach the
same target. A single-connection session can be established to any
of these TCP addresses. A multiple-connection session could span
addresses .45 and .46 or .47 and .48 but cannot span any other
combination. A TargetAddress with its own tag (.49) cannot be
combined with any other address within the same session.

This SendTargets response does not indicate whether .49 supports
multiple connections per session; it is communicated via the
MaxConnections text key upon login to the target.

Appendix D. Algorithmic Presentation of Error Recovery Classes

This appendix illustrates the error recovery classes using a
pseudo-programming language. The procedure names are chosen to be
obvious to most implementers. Each of the recovery classes described
has initiator procedures as well as target procedures. These
algorithms focus on outlining the mechanics of error recovery classes
and do not exhaustively describe all other aspects/cases. Examples
of this approach are as follows:

Resultant cases, such as recovery of Synchronization on a header
digest error, are considered out of scope in these algorithms.
In this particular example, a header digest error may lead to
connection recovery if some type of Sync and Steering layer is
not implemented.

These algorithms strive to convey the iSCSI error recovery concepts
in the simplest terms and are not designed to be optimal.

D.1. General Data Structure and Procedure Description

This section defines the procedures and data structures that are
commonly used by all the error recovery algorithms. The structures
may not be the exhaustive representations of what is required for a
typical implementation.

The initiator algorithms only deal with unsolicited NOP-In PDUs for
generating Status SNACKs. A solicited NOP-In PDU has an assigned
StatSN that, when out of order, could trigger the out-of-order
StatSN handling in within-command algorithms, again leading to
Recover-Status-if-Possible.

The pseudocode shown may result in the retransmission of
unacknowledged commands in more cases than necessary. This will
not, however, affect the correctness of the operation because the
target is required to discard the duplicate CmdSNs.

- The procedure Build-And-Send-Async is defined in the connection
recovery algorithms.
- The procedure Status-Expect-Timeout-Handler describes how
initiators may proactively attempt to retrieve the Status if they
so choose. This procedure is assumed to be triggered much before
the standard ULP timeout.

Transport exception conditions such as unexpected connection
termination, connection reset, and hung connection while the
connection is in the Full Feature Phase are all assumed to be
asynchronously signaled to the iSCSI layer using the
Transport_Exception_Handler procedure.

Appendix E. Clearing Effects of Various Events on Targets

E.1. Clearing Effects on iSCSI Objects

The following tables describe the target behavior on receiving the
events specified in the rows of the table. The second table is an
extension of the first table and defines clearing actions for more
objects on the same events. The legend is:

Y = Yes (cleared/discarded/reset on the event specified in the row).

Unless otherwise noted, the clearing action is only applicable
for the issuing initiator port.

connection in question and waiting to be reordered in the
iSCSI layer. All "Y"s in this column assume that the task
causing the event (if indeed the event is the result of a
task) is issued as an immediate command, because the
discontiguities can be ahead of the task.

(2) Discontiguous Data (DD) are data PDUs received for the task in

question and waiting to be reordered due to prior
discontiguities in the DataSN.

(3) "SS" refers to the StatSN.

(4) "CS" refers to the CmdSN.

(5) "DS" refers to the DataSN.

(7) This action clears the StatSN on all the connections.

(8) This sequence number is instantiated on this event.

(9) A logout failure drives the connection state machine to the

CLEANUP_WAIT state, similar to the connection failure event.
Hence, it has a similar effect on this and several other
protocol aspects.

(10) This is cleared by virtue of the fact that all sessions with

all initiators are terminated.

(11) This clearing effect is "Y" if it is a connection

reinstatement.

(12) This clearing effect is "Y" only if it is a connection

reinstatement and the operational ErrorRecoveryLevel is 2.

(13) This clearing effect is "N" only if it is a connection

reinstatement and the operational ErrorRecoveryLevel is 2.

E.2. Clearing Effects on SCSI Objects

The only iSCSI protocol action that can effect clearing actions on
SCSI objects is the "I_T nexus loss" notification (Section 6.3.5.1
("Loss of Nexus Notification")). [SPC3] describes the clearing
effects of this notification on a variety of SCSI attributes. In
addition, SCSI standards documents (such as [SAM2] and [SBC2]) define
additional clearing actions that may take place for several SCSI
objects on SCSI events such as LU resets and power-on resets.

Since iSCSI defines a TARGET COLD RESET as a "protocol-equivalent" to
a target power-cycle, the iSCSI TARGET COLD RESET must also be
considered as the power-on reset event in interpreting the actions
defined in the SCSI standards.

When the iSCSI session is reconstructed (between the same SCSI ports
with the same nexus identifier) reestablishing the same I_T nexus,
all SCSI objects that are defined to not clear on the "I_T nexus
loss" notification event, such as persistent reservations, are
automatically associated to this new session.

Acknowledgments

Several individuals on the original IPS Working Group made
significant contributions to the original RFCs 3720, 3980, 4850,
and 5048.

Specifically, the authors of the original RFCs -- which herein are
consolidated into a single document -- were the following: